mime-settings: Allow setting default application for multiple MIME types

Hello!

Usecase:

1. User has "GNOME videos" set to multiple mime types
2. Now user wants to set "VLC" for multiple mime types

Currently, user can set "VLC" as default application one by one only which is cumbersome.

We should allow the user to select multiple mime types and setting default application for those selected mime types.

Thanks!