Minesweeper: Add "Custom game..." difficulty

This adds a dialog window which allows us to customize the size of the
board and the amount of mines that will be placed.
The current max amount of mines is 50% of the total number of cells
due to the fact that the generator algorithm takes too long to create a
board for higher percentages of mines.

Userland/Games/Minesweeper/MinesweeperCustomGameWindow.gml

@@ -0,0 +1,80 @@
+@GUI::Widget {
+    fill_with_background_color: true
+
+    layout: @GUI::VerticalBoxLayout {
+        margins: [4]
+    }
+
+    @GUI::GroupBox {
+        title: "Field"
+        autosize: true
+
+        layout: @GUI::HorizontalBoxLayout {
+            margins: [16, 6, 6]
+        }
+
+        @GUI::Label {
+            text: "Columns: "
+            autosize: true
+        }
+
+        @GUI::SpinBox {
+            name: "columns_spinbox"
+            min: 10
+            max: 50
+            fixed_width: 40
+        }
+
+        @GUI::VerticalSeparator {
+        }
+        
+        @GUI::Label {
+            text: "Rows: "
+            autosize: true
+        }
+
+        @GUI::SpinBox {
+            name: "rows_spinbox"
+            min: 10
+            max: 50
+            fixed_width: 40
+        }
+
+        @GUI::VerticalSeparator {
+        }
+
+        @GUI::Label {
+            text: "Mines: "
+            autosize: true
+        }
+
+        @GUI::SpinBox {
+            name: "mines_spinbox"
+            min: 1
+            max: 2500
+            fixed_width: 50
+        }
+    }
+
+    @GUI::Widget {
+        max_height: 24
+
+        layout: @GUI::HorizontalBoxLayout {
+        }
+
+        @GUI::Widget {
+        }
+
+        @GUI::Button {
+            name: "ok_button"
+            text: "OK"
+            max_width: 75
+        }
+
+        @GUI::Button {
+            name: "cancel_button"
+            text: "Cancel"
+            max_width: 75
+        }
+    }
+}