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 think this is fine as well. The code looks much more readable now, so thank you!!  This does not affect us right now, but calling showline twice will probably add the inline widget twice.  But for now this is good.