Add 'Select All' option

Add new menu entry `Select All` in `Edit` menu bar.
As usual use the short-cut `ctrl` + `a` to trigger it.

This action should select all shapes on the graphic scene.

Start at the Edit menu construction and the graphic-scene selection handling, then trace how keyboard shortcuts are registered. Add a Select All menu action bound to Ctrl+A that selects every shape in the scene, and verify both the menu command and shortcut produce the expected selection.

Written by the indexing model from the issue text.