navigator: run initial watcher adds in background
navigator: run initial watcher adds in background

Running vidar in wine on linux (to test windows features), it was taking
literal minutes to start up because the watcher code was taking so long.
This punts that logic to the background so that the editor will be useable
while it's still setting up watches.

nelsam/gxui_playground,nelsam/vidar,Kvaz1r/vidar