Issue 5975069: Ignore mouse masks in ui/gtk3/keybindingmanager for accessibility.

Keyboard Shortcuts

	File
u :	up to issue
m :	publish + mail comments
M :	edit review message
j / k :	jump to file after / before current file
J / K :	jump to next file with a comment after / before current file
	Side-by-side diff
i :	toggle intra-line diffs
e :	expand all comments
c :	collapse all comments
s :	toggle showing all comments
n / p :	next / previous diff chunk or comment
N / P :	next / previous comment
<Up> / <Down> :	next / previous line
<Enter> :	respond to / edit current comment
d :	mark current comment as done

	Issue
u :	up to list of issues
m :	publish + mail comments
j / k :	jump to patch after / before current patch
o / <Enter> :	open current patch in side-by-side view
i :	open current patch in unified diff view

	Issue List
j / k :	jump to issue after / before current issue
o / <Enter> :	open current issue
# :	close issue

	Comment/message editing
<Ctrl> + s or <Ctrl> + Enter :	save comment
<Esc> :	cancel edit