Emmet usability fixes:

- fix balloon width
- show last selected abbreviation by default
- request focus properly
- fix NPE on dispose
(cherry-picked from 44cbcd6)
UI: add isComboPopupKeyEvent utility method

related issue: WEB-9744
(cherry-picked from 445de2c)
[r=kudryashov]
WEB-9744 Surround with Emmet dialog usability problems
(cherry-picked from 1b92d0c)