cleanup various kinds of properties

We have several kinds of properties (GObject properties, xfconf settings, etc) that often sound similar, but are actually entirely separate domains. Sometimes we already have define's for them, sometimes those aren't used everywhere, sometimes scattered across several places, etc.

This patch queue gives this a bit cleanup, separate prefixes, consolidating definitions in once place, etc, in order to make the code a bit easier to read / understand. Mostly just preprocessor-visible changes, shouldn't change the compile output much.