Fix gettext setup in test_common and log in test_transfer so tests run standalone

Add extra tests to test_config
Run the test files through flake8 to tidy up code
Handle plugin info not available
Removed call to save_config in path-chooser (obsolete)
Fixup saving and loading state files

 * All state files have a backup created before saving
 * The backup will now be used if saving or loading fails
 * GTKUI state files stored in new gtkui_state dir and common load/save functions created
 * Detects bad shutdown and archives timestamped state files in separate config directory.