Implemented the deletion function

Closes #17

I18N Commander/Processor/SectionProcessor.cs

@@ -82,4 +82,30 @@ public static class SectionProcessor
         await db.SaveChangesAsync();
         return section;
     }
+
+    public static async Task RemoveSection(DataContext db, string selectedKey)
+    {
+        // Remove the section from the database:
+        var section2Delete = await db.Sections.FirstOrDefaultAsync(n => n.DataKey == selectedKey);
+        if (section2Delete is null)
+            throw new ArgumentException($"The section with key {selectedKey} does not exist in the database.");
+        
+        // Next, remove all children of the section, and the children's children, etc.:
+        var children = await db.Sections.Where(n => n.Parent == section2Delete).ToListAsync();
+        foreach (var child in children)
+            await RemoveSection(db, child.DataKey);
+
+        db.Sections.Remove(section2Delete);
+        await db.SaveChangesAsync();
+    }
+    
+    public static async Task<int> NumberChildren(DataContext db, string selectedKey)
+    {
+        // Read the section from the database:
+        var section = await db.Sections.FirstOrDefaultAsync(n => n.DataKey == selectedKey);
+        if (section is null)
+            throw new ArgumentException($"The section with key {selectedKey} does not exist in the database.");
+        
+        return await db.Sections.CountAsync(n => n.Parent == section);
+    }
 }

I18N Commander/UI WinForms/Components/SectionTree.Designer.cs

@@ -91,6 +91,7 @@
             this.buttonRemove.Text = "Remove";
             this.buttonRemove.TextImageRelation = System.Windows.Forms.TextImageRelation.ImageBeforeText;
             this.buttonRemove.UseVisualStyleBackColor = true;
+            this.buttonRemove.Click += new System.EventHandler(this.buttonRemove_Click);
             // 
             // treeView
             // 
@@ -102,6 +103,7 @@
             this.treeView.Name = "treeView";
             this.treeView.Size = new System.Drawing.Size(290, 439);
             this.treeView.TabIndex = 1;
+            this.treeView.NodeMouseClick += new System.Windows.Forms.TreeNodeMouseClickEventHandler(this.treeView_NodeMouseClick);
             // 
             // SectionTree
             // 

I18N Commander/UI WinForms/Components/SectionTree.cs

@@ -31,10 +31,10 @@ public partial class SectionTree : UserControl
         this.treeView.ImageList = imgList;
 
         // Subscribe to the load event:
-        this.Load += this.OnLoad;
+        this.Load += this.LoadNodes;
     }
 
-    private async void OnLoad(object? sender, EventArgs e)
+    private async void LoadNodes(object? sender, EventArgs e)
     {
         // A dictionary to cache all known tree nodes:
         var treeNodes = new Dictionary<string, TreeNode>();
@@ -146,4 +146,37 @@ public partial class SectionTree : UserControl
         node.EnsureVisible();
         this.treeView.SelectedNode = node;
     }
+
+    private async void buttonRemove_Click(object sender, EventArgs e)
+    {
+        // Get the currently selected section, which will be removed:
+        var selectedNode = this.treeView.SelectedNode;
+        
+        // Get the number of children:
+        // (notice, that the node's name is its key)
+        var numberChildren = await SectionProcessor.NumberChildren(this.db, selectedNode.Name);
+        
+        // Ask the user, if he really wants to remove the section:
+        if(MessageBox.Show(numberChildren > 0 ? $"Are you sure, you want to remove the section '{selectedNode.Text}', its {numberChildren} children and so on?" : $"Are you sure, you want to remove the section '{selectedNode.Text}'?", "Remove section", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.No)
+            return;
+        
+        // Remove the section from the database:
+        // (notice, that the node's name is its key)
+        await SectionProcessor.RemoveSection(this.db, selectedNode.Name);
+        
+        // Remove all nodes from the tree control:
+        this.treeView.Nodes.Clear();
+        
+        // Reload the tree:
+        this.LoadNodes(this, EventArgs.Empty);
+    }
+
+    private void treeView_NodeMouseClick(object sender, TreeNodeMouseClickEventArgs e)
+    {
+        // Get the currently selected section:
+        var selectedNode = this.treeView.SelectedNode;
+        
+        // If the selected node is not null, enable the remove button:
+        this.buttonRemove.Enabled = selectedNode is not null;
+    }
 }