Add useful shortcuts to TUX editor

All buttons to insert/replace text (insertables and then translation aids
in order; "discard changes" is skipped) are numbered from 1 up and can be
accessed with both alt+<num> and alt+shift+<num>.

Discovery of the shortcuts is left as an exercise to the user.

Bug: 45479
Change-Id: Icf593794936dfc40bbbb1d621c11dc7b2d90e3ba
Merge "Add useful shortcuts to TUX editor"