Add Super+F for Files (while keeping "Explorer" shortcut)

[cassidyjames]

Since someone's keyboard (like my Logitech!) might have the "Explorer" key, we should keep it in addition to adding Super + F to launch Files

[jbicha]

In case you hadn't already discovered this, my understanding is that the media-keys shortcuts are limited to only one keyboard shortcut. Because there isn't a good way to migrate gsettings, this is difficult to fix well. See https://bugzilla.gnome.org/show_bug.cgi?id=677427

[cassidyjames]

@jbicha this appears to be an old issue; we ended up just keeping the Explorer shortcut due to your mentioned reasons. @jackpot51 mentioned that we could pre-set a custom shortcut for Super + F for files, but we weren't sure if that felt kosher or not.