Prevent clicks on a focused editor from unfocusing its hidden input

This closes #233. When an editor is focused, what's *actually* focused
is a hidden input element inside the editor. So clicking on a focused
editor was temporarily stealing focus away from the hidden input, and
then moving it back to the hidden input (which the editor always does
when it is focused). Returning false from the mousedown handler when
the editor is already focused prevents this bouncing of focus and
prevents the fuzzy finder from hiding when you click its editor.

src/app/editor.coffee

@@ -379,6 +379,7 @@ class Editor extends View
       @overlayer.show()
       e.target = clickedElement
       $(clickedElement).trigger(e)
+      false if @isFocused
 
     @renderedLines.on 'mousedown', '.fold.line', (e) =>
       @destroyFold($(e.currentTarget).attr('fold-id'))