pothos-gui: better job with extra shortcuts to avoid duplicates
pothos-gui: move block search match into its own method
pothos-gui: created block cache for fetching and storing block desc

The block tree window is now signaled by the cache on update.
The cache can be signaled to refetch.
TODO adding/removing nodes should cause a refetch.
Also we switched to passing JSON Objects when possible.
pothos-gui: global widget map is now object map