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"? :)
I would prefer not to have to set a property for the line number, but although `widget.info.line` returns the `CodeMirror.Line`, I can't find a way to get the line number from that.