Detect mounted devices using system capability on Linux

[acolombier]

Currently on Linux, mounted devices are detected by listing directory in /media or /var/media. This isn't ideal as:

1. User may have defined custom mount directory (e.g using udisk) for specific devices
2. System may have configuration to mount device somewhere else

Options for improving this could be to:

1. Parse the content of /proc/mounts / result of df commands (easy, not able to detect available but not yet mounted devices)
2. Integrate with udisk dbus (complete but significant amount of work)