pothos-gui: context deleter for gui blocks
pothos-gui: fix orphan delete loop and uniqueness
pothos: new render modes for topology dot markup
pothos-gui: added future for dot render in bg
pothos-gui: switch to pixmap, and set default pixmap
pothos-gui: no thread pool for graphical blocks
pothos-gui: fix handling of save-as cancel result
pothos-gui: show affinity zone for gui blocks in properties panel