app: add gimp-tool-options-manager.[ch]
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.
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