Contextual help for Script Editor

- Press Shift + F1 to display help for selected text or word under cursor
- Add "get_word_under_cursor" to TextEdit api
- EditoNode only consume F1 key when no Shift or Command been pressed

Known issue:
When help tab been invoked first time, it scroll to wrong position. After the that, it works as expected.

See post http://www.godotengine.org/forum/viewtopic.php?f=8&t=546&start=10#p2317 for more info.