Multiple file/folder selection support

This (large) commit adds support for selecting multiple files/folders
for the following operations:

* Import files/folders
* Export files/folders
* Delete files/folders
* Cut/Copy and paste files/folders

The multiple selection mode is triggered via long pressing on a file
name. A multiple item chooser interface replaces the previous context
menu. Upon making their selections the user can then choose the action
they want to invoke using the CAB (context action bar) interface.

The progress display mechanism was somewhat reworked to display a more
useful progress indication, although further work is needed to improve
on it. Currently we can show individual file names in all multiple
selection operations which can give the user a good sense of where the
long running operation is at.