The way we currently manage tool options, and particularly copy things
around for "global_brush", "global_pattern" etc. sucks, is spread
across files, happens only on tool change, is thus buggy and leads to
all sorts of small inconsistencies.

This new manager will replace all of that stuff, and it does it in one
place, and will keep the user context, the global paint options, and
all tool options connected permanently, and only connect/disconnect
things when settings change, so everything is always in a consistent
state.

Pushed for review only, nothing is used yet.