This is big, but actually mostly straightforward.

The panel properties dialog still needs to be ported, and there is some
code commented out (with #if 0) to enable build without porting this
yet.

We needed to split some enums in a new file (panel-enums-gsettings.h),
so that we can generate GSettings schemas for them and use enums for
keys.

Note that we also drop the opacity key for background, as we directly
store RGBA in the color key now.