File browser 'BOOT BIOS' now optional. Can set to also display 'SELECT CURRENT FOLDER' instead using fragment arguments. Starting the emu is now handled by the fragment container instead of the FileBrowser class. Class passes cd image URI to fragment callback when image is selected. Likewise, 'SELECT CURRENT FOLDER' will pass the CWD as a URI to another callback in the fragment container. This is in preparation for re-using the file browser for the options activity.