orientation: Unown the D-Bus name on Stop

Instead of when finalizing the plugin, so that stop and starting
the plugin again works as expected.
orientation: Use the G_SOURCE_REMOVE define

Instead of just FALSE, this is a cosmetic change.
orientation: Unset source ID on idle callback

The idle was being removed, but the source ID was left != 0, so on
shutdown it'd cause warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=719973
orientation: Fix plugin on MS Surface devices

When plugging in the type/touch cover on a Microsoft Surface, the
IIO sensor is unplugged, and plugged in again with a different
sysfs path. As the accelerometer comes and goes, we need to handle this
in the orientation plugin, and not bail out if we can't find the
accelerometer on startup, and handle devices coming and going.