Filter ISO files in file chooser of Tails Installer

The user has to go the full path every time wants to select a iso file.
The tails installer should remember the last directory used.

Feature Branch: tails:feature/12705-bump-system-partition-size installer:bug/12436-remember-iso

Original created by @kurono on 12436 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information