Improve focusType detection

Improve focusType detection

Inputs focused and blurred inside the Evernote Web Clipper extension's
popup (which is an `<iframe>` injected into the browser UI) were
previously not detected (making it difficult to type into them without
triggering VimFx commands). That was because we have to check if events
really are UI events or not in order to support non-multi-process
Firefox (which expose web page content events to UI code), and
Evernote's elements seem to be indistinguishable from web page content.

This commit simply removes the check for where the event comes from when
looking for focusType changes. Luckily, it doesn't matter if we happen
to handle web page content events in the UI.

To avoid excessive 'focusTypeChange' events (a single focus or blur
might trigger 'focusTypeChange' twice now: Once in the UI code, and once
in the frame script), this commit also makes sure that setting the
focusType to the same value as before is a no-op.

Fixes #733.