add Peppermint 7 support

[s4w3d0ff]

I get:

Couldn't detect your desktop environment ('unknown'), but you have 'nitrogen' installed so we will use it...
Could not get bg groups from config file.

In python 3:

>>> import os
>>> os.environ.get("DESKTOP_SESSION")
'Peppermint'
>>> os.environ.get("XDG_CURRENT_DESKTOP")
'LXDE'

Haven't tested yet but it seems like this should do the trick.

[s4w3d0ff]

this patch is now tested and works on Peppermint 7

[boramalper]

Thanks a lot! =)