Double-click a shape on canvas to edit its label

Allow double-clicking an existing shape on the canvas to open the label-edit dialog directly.

Origin: PR #1361. Not present in main today. A clean implementation should hook Qt's native mouseDoubleClickEvent in labelme/widgets/canvas.py to emit a signal (analogous to selection_changed) that app.py connects to _edit_label, rather than the PR's manual time-delta double-click detection.