Allow box selecting over CanvasItems when Shift is held

[KoBeWi]

See title.

godot.windows.editor.dev.x86_64_gdtlDepF5d.mp4

This is very useful after #92016, as box selecting over a TileMapLayers is no longer possible, so selecting nodes over tiles became more difficult.

[Calinou]

Tested locally, it works as expected (including the drag distance threshold behavior).

This mimics behavior found in Inkscape (also using Shift), which makes sense.

[akien-mga]

Thanks!