update_views takes a very long time to complete

Currently we use update_views to refresh data that is displayed in all iconviews. It is very slow, because it cleans up entrire treemodels, and fills them back again. Depending on machine specs, it's execution can take up to 0.5 seconds. It is also frequently called when not needed. This makes the interface feel very laggish, especially when resizing the window or using filtering features.

If we migrate all filtering features to gtk filters, and remove the need to clear & refill whole treemodels (most likely giving up on update_views to a set of more efficient calls), not only the application will be much more respondive, but also the code will be much cleaner and well-structured.