LXDE (Pcmanfm) support (WIP)

[daf]

Fixes #63

Adding setter backend for Pcmanfm (aka what LXDE uses).

TODO:

• Clean up exception pathways
• Make sure Currently Set strings in UI make sense
• Don't prompt for saving/dirty flag
• Make sure configs don't crash pcmanfm!

Since Pcmanfm's command line switch can't take a monitor parameter in any way, it always defaults to monitor 0, which means Nitrogen can't control the other heads as appropriate. Therefore, we directly manipulate the Pcmanfm desktop configuration files and send it a SIGUSR1 which seems to reload it. I'm not sure this is an incredibly stable approach, though.

[daf]

There's just not a lot I can do about stability with the SIGUSR1 approach, I can't tell even what does the restart in an LXDE session. We're going to call this a "beta" feature :)