Speed up loading large folders

[jeremypw]

Addresses #1794

This PR uses two strategies for speeding up loading/model creation

• Add the initially loaded files in one hit in the done-loaded signal handler instead of individually
• Create the file-treerow map asynchronously after the model has loaded.

These strategies decrease the time that the UI is frozen significantly, particularly for ListView.
For IconView the effect is less marked because the majority of the time is taken in actually laying out the view; this largely occurs inside Gtk.IconView and still freezes the UI. There is little that can be done about this until IconView is replaced.

To load and display 20,000 small files on my system
ListView: 22 seconds -> 3 seconds
IconView: 52 seconds -> 29 seconds