Fix repeated mode changes

Make sure to properly undo and redo all the setup
work in setup_indicator and remove_indicator so they
can be used repeatedly.
Port the overlayscroll example

Make the overlayscroll example use the overlay scrolling that is
now built into GtkScrolledWindow, instead of the manual prototype.
Theme improvements

Make scrollbars in sidebars show up - this what broke the gtk3-demo
scrollbar.