Fix focus handling

Fix focus handling

When blurring a UI element, we handle that blur after a timeout. If
the UI element was blurred due to focusing a frame element, the focus
events for that frame element occurs _before_ handling the UI blur. That
caused the focusType to be set according to the UI blur, and not
according to the frame focus. This meant that if you had the location
bar focused and clicked inside a text input in a web page, VimFx would
enter and then immediately exit "auto insert mode". This commit fixes
this issue by asking the frame script for the focusType if a 'focus' or
'blur' event is caught in the UI and the active element is the current
`<browser>` (instead of treating `<browser>` as focusType 'none').

Also, since commit fee33fa8, 'focusTypeChange' events are only sent when
_changing_ the focusType of a `vim`, _not_ when trying to set the
focusType to the same value as before. While that's good, it caused a
problem when switching tabs. When switching tabs, 'blur' events occur in
the old tab and (re-)'focus' events occur in the new tab, but no
focusType of any `vim` is necessarily changed. However, one tab might be
in "auto insert mode" while the other isn't, so the button would still
need to be updated. Therefore, this commit sends a 'focusTypeChange'
event when switching tabs.

See #738.