[Bug] add option to mount an external device

[feikedonia]

When I connect* my external harddrive, kde Dolphin gives me the option to mount it. Cosmic files does not.
(* The drive isn't yet mounted, but it is in /dev )

Dolphin detects my external drive Reservebestanden while Cosmic Files doesn't

When I mount the external drive via the Dolphin file manager, Cosmic Files also detects the drive, after some half a second.

Cosmic version alpha 4,
Endeavouros.

May be related to #480