Add keybind and change custom inline widget to Brackets' inline widgets.

Fixes #38.

 - Changes custom inline widget to use default inline widgets
 - Ctrl/Cmd+Shift+E opens the descriptions of the errors on current line.

@peterflynn: Could you check my implementation of inline widgets to make sure I'm not doing anything "bad"? :)
@Mark-Simulacrum Fwiw, if there's already a widget on the same line `addInlineWidget()` is supposed to close that existing widget and then proceed with opening the new one.  I don't think it would ever no-op.