Move mount/unmount/eject actions into launcher (Issue #349)

Provides the possibility to reuse device-related menu
services by the bookmark view, instead of prvoding own code.

Fixes #349
2 jobs for !26 with ThunarMenuTreeView3 in 6 minutes and 7 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Build
passed #2557
build

00:02:38

 
  Distcheck
passed #2558
distcheck

00:03:29