Add support for canvas layouts and blank spacer zones

[ChaoticGood007]

Overview

This PR implements support for custom Canvas / Overlapping Layouts (type: 2) in the gSnap layout editor, allowing users to define freely-positioned, overlapping window regions alongside standard Grid layouts. It adds a native dropdown dialog selector to choose the layout type when creating new layouts, drag-and-drop movement, edge/corner resizing, double-click zone creation, standard splitting/editing controls, and customizable overlap snapping preferences.

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Key Features Added

1. Canvas / Overlapping Layout Type (type: 2)

• Extends the layout schema with a canvas layout option (type: 2) supporting absolute coordinates (x, y, w, h) instead of traditional grid constraints.
• Allows layouts to contain overlapping regions, freely placed anywhere in the work area.

2. Interactive Canvas Editor

• Drag-and-Drop Moving: Drag anywhere in the center of a canvas zone to move it.
• Edge/Corner Resizing: Drag borders (ns-resize, ew-resize) or corners (nwse-resize, nesw-resize) to resize zones with appropriate cursor feedback.
• Double-Click to Spawn: Double-click on any empty space inside the editor background to instantly spawn a new zone.
• Standard Controls: Left-Click a zone to split it vertically, Middle-Click to split it horizontally, and Right-Click to remove/delete it.

3. Layout Type Selection Dropdown in Creation Dialog

• Adds a native GNOME Shell PopupMenu dropdown menu selector to the layout creation dialog (where previously only layout naming was supported).
• When creating a new layout, users can choose between "Grid Layout" and "Canvas Layout".
• The selector is integrated into the dialog with the label "Layout Type:" on the left and the selector button on the right.
• Connected a PopupMenuManager to manage overlay focus and prevent pointer/click lockouts.
• Ensures all popup actors are properly disposed of on cancel/confirm click to avoid memory leaks in Main.uiGroup.

4. Overlap Snapping Strategy Preference Setting

• Adds a new preference option under the basics settings tab (Overlap Snapping Strategy) to configure how overlapping zones are matched:
  • First Match (default): Snaps to the first matched canvas zone.
  • Closest Center: Snaps to the zone whose center coordinate is mathematically closest to the mouse pointer, avoiding awkward bounding box merges during overlapping zone dragging.

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Technical Implementations

• Cursor Constraints: Clamps drags to keep zones inside the bounds of the work area monitor, ensuring canvas layout items are never moved off-screen.
• Typings Expansion: Added GObject mapping for cursor_type to StWidget inside gnometypes.ts for clean TypeScript compilation.
• Documentation: Updated README.md to detail both Grid and Canvas editing controls.

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Verification & Testing

1. Click the gSnap panel indicator and choose Create New Layout.
2. Select Canvas Layout from the new dropdown selector.
3. Open the Edit Layout mode:
   • Double-click empty canvas space to add zones.
   • Resize and move zones using mouse drags.
   • Save layout and verify window snapping works correctly.