Rename "Linux Devices" to "Devices" in settings GUI.

We will soon have non-Linux devices in there as well. Consequently, the
whole device configuration infrastructure should be refactored and a
base class be moved to the core plugin in a follow-up commit.

Change-Id: I35cfa9e020dd42856fd3c25308cdff7a65d3887b
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>