Theme changes to sync with libadwaita

These changes have been discussed in the recent libadwaita meeting, and a bit on irc afterwards.

The current favorite plan for how to deal with themes when we have libadwaita as GNOME platform / hig library appears to be roughly the following:

• Copy Adwaita, Adwaita-dark, HighContrast, HighContrastInverse to libadwaita
• Extend them to cover whatever extra widgetry ends up in that library (e.g. add selection mode)
• Rename the themes in GTK to something else (suggestion: Default -light,-dark, -highcontrast, -highcontrast-dark)
• Simplify the themes in GTK over time (e.g. drop linked buttons)