Add 'Mount' option to context menu of unmounted device in Places

Summary:
This is only a proposal but:
1/ it the sysmetric of the Unmount entry in the context menu of mounted devices
2/ I have to admit it took me a very long time to discover (by accident) that you can just (left-)click on a unmounted device to get it mounted automatically. I might not be the only feeling that you should be able to mount a device from the context menu ...

Twin of D15988 in Kio while dolphin is not based on the kio code for Places.

{F6306516}

Reviewers: #dolphin, #vdg, ngraham, elvisangelaccio

Reviewed By: #dolphin, #vdg, ngraham, elvisangelaccio

Subscribers: ngraham, acrouthamel, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D15974