Finish revamping the view item defaults system. There is now a Settings->Theme dialog
which sets brush, stroke, and font for all view items, optionally applying to all existing
objects, and/or saving as the default for all future objects.

Furthermore, all view objects can save their current status as the default for that
particular type from their own dialogs.

Tabs can now have their own background, on a per tab basis.

It is now easy to change an entire kst session from black on white to white on black.