When choosing a panel, check which side is focused

This prevents a rare weirdness where if both sides are showing the same
pane, and a user action on the left side initiates a change to a
different view in the same pane (e.g. a different Index page), the view
would change on the right side instead. Now it changes on whatever side
had the focus.
Rename Game pane to Story pane

Do the renaming in the code and comments as well, so that everything
remains readable.
Rename Results/Problems tab to Results/Report

Also carry through the renaming in the code and comments so that
everything remains readable.
Refactor the Settings pane to use GBinding

...at least, wherever possible. It's not possible for the radio button
group, and it's not (yet) possible because I7SourceView's
elastic-tabstops setting isn't implemented as a GObject property. Cuts
out a lot of code.
Small refactoring to reduce code repetition