Extracts out the creation of an NSEvent from a WidgetKeyEvent in
TextInputHandler.mm into generic helper method. The helper is used by headless
to create a fake NSEvent to then build edit commands from key events.
Fixes:
- test_selectevents.html
- test_bug756984.html
- test_movement_by_characters.html
- test_movement_by_words.html
- test_backspace_vs.html
- test_bug1094000.html
- ... many key event tests
MozReview-Commit-ID: 1Jur5MHOrkp
