The systray icon will animate so long as a gateway+folder tuple is found in the gui.core.operations list. This one-line PR updates Model.remove_folder to also call Model.on_syncing_finshed, thereby ensuring that removed folders are always also removed from the operations list -- and thereby stopping the systray icon from endlessly spinning/animating in the event that the folder gets removed before the syncing operation naturally completes.

Fixes #197