Load selected tab

Store (and reload) currently selected tab in savegames.
Open multiselection tab on load if more than one instance was selected.
See #2165.

This only happens for single-instance selections. The build menu and
other tabs or windows are not restored upon loading games.

We make an exception here because
* the selected instances themselves are already stored
* they are "selected" upon loading a savegame

Until very recently however, this selection did not show any
corresponding tab, just the colored range highlights.
I believe keeping track of what was selected in a savegame is correct,
player-expected behavior. We show the tab menu now, so pulling up
the right tab is not far off. As a bonus, it means we can save and
reload a game in gui tests and expect these tabs to still be there.