Implement Backspace key support for polygon/polyline creation

[dhruv1955]

Summary

Add Backspace key support to remove the last placed point during polygon
and polyline creation, without canceling the entire shape.

Problem

When drawing polygons or polylines, users had no way to undo just the last
placed point. The only option was Escape, which canceled the entire shape —
forcing a restart even for a single mis-click.

Solution

Handle Qt::Key_Backspace in keyPressed() when state() == CreatingObject:

• Cancels the operation if only the starting point remains
• Removes the last point (or first, when extending from the beginning)
• Creating mode: updates polygon directly via setPolygon()
• Extending mode: pushes ChangePolygon onto the undo stack for proper history
• Calls synchronizeOverlayObject() to refresh the visual preview

Changes

• File: src/tiled/createpolygonobjecttool.cpp
• Function: keyPressed() — previously only had TODO comments

Testing

1. Select Insert Polygon tool (P), click 3–4 points, press Backspace — last point removed
2. Press Backspace repeatedly until 1 point remains — operation cancels
3. Extend an existing polyline, add points, press Backspace — point removed with undo history entry

Notes

Resolves the first TODO in keyPressed(). The second TODO
(Shift+Enter to finish as polygon) is left for a follow-up.