Typo fix
GtkStack: Improve focus handling

Add notebook-like focus handling: Keep track of the last focused
descendent of each page, and focus it again when switching back
to the page. If there is no last focused child, we move the focus
into the page as if the user had hit Tab.
inspector: Fix focus in css editor

Give the text view initial focus, so things work as expected
when switching to the css editor for the first time.
inspector: Make sure something is focusable on the stats page

Without this, the focus will not be inside the top_stack when
we switch to the statistics page and it shows the excuse. This
in turn will make the next page not take focus as expected when
switching away from the statistics page again.
inspector: Make header buttons as !focus-on-click

Moving the focus to the header bar is unexpected and disruptive,
so don't do it.