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 actually don't think it will -- it may cause the widget to close and open, but nothing more than that: `addInlineWidget `on the `Editor` actually (as far as I can tell) checks whether or not there is a widget on that line already, and doesn't do anything if there is.

The reason we need custom logic to remove a widget if it is on that line, I think, is because the `addInlineWidget` function won't close the widget if it is already open (and we want to do that).