Should have obvious way to close inline editors

[njx]

There's no visible UI for closing an inline editor--you have to know to hit Esc. (In some editors hitting Cmd-E again closes it, but apparently that doesn't work in all editors right now: https://groups.google.com/forum/#!topic/brackets-dev/YiDl6MBCzpo.) We should have an "X" or some such for closing the editor, and also make sure that Cmd-E consistently closes the editor.

[njx]

To @larz0 to consider visible UI for this.

[RaymondLim]

FBNC to @njx

[njx]

This looks good everywhere except Quick Docs. I'll open a separate bug for that.