Add keyboard shortcut for 'clear' button

Now Ctrl+\ (Cmd+\ on Mac) will trigger the 'clear annotations' button
on the current context. Ideally we'd also bond to the 'clear' keyboard
button (ASCII 12) but it does not seem possible to do that yet.

Bug: 51507
Change-Id: I300ec1ffa237e51418ec429be39001f820f053ae
Merge "Add keyboard shortcut for 'clear' button"