Hard to find an "inspectable" widget when in widget selection mode

[elliette]

When widget selection mode is on, it is hard to figure out where to click to actually "select" a widget.

This could be made easier if bounding boxes were shown on hover, similarly to what Chrome Devtools does:

[kenzieschmoll]

Tagging some related issues here:

• Highlight selected widget on device screen without entering the on-screen selection mode #3926 (duplicate but has some specific feedback from user survey)
• Prototyping a continuous on-screen widget selection mode #3567
• For Flutter Web, provide a chrome right click menu item to inspect.  #3712
• Use same in-app toggle select widget mode as in Widget Inspector  #4160