appimaged: Handle opening AppImages when Firejail is installed

When downloading files using the browser, then opening the downloaded AppImage through the browser does not work (because when `firejail` is available, then `appimaged` does not set the executable bit).

This could be changed by registering the AppImage type-1 and type-2 MIME types with a helper script that would run the AppImage file using firejail.

Potentially the functionality could even be added to appimaged which already has most of the logic for determining how to run an AppImage with firejail in place - pull requests welcome!