Finalize service configuration on GUI side

Add folders selector, install/uninstall service button.
Also make the service a bit safer by checking witch folders actually exist.
Don't watch the same folder twice

Ignore the folder if it's either watched directly or as a subfolder from some other watched folder
Don't add the same folder twice in the GUI

Still allowing to add children folders that might already be watched because user could change WatchSubfolder later at some point and probably doesn't want to lose manually added children.
Re-create watchers when reloading settings
Don't reload settings twice on one change

"Changed" event fires multiple times while settings are being written
Update readme