Add lxde. (Mixed content: 4 updates, 7 RFCs)

[contact . ng0]

[PATCH 01/11] gnu: lxterminal: Update to 0.3.0.
[PATCH 02/11] gnu: pcmanfm, libfm: Update to 1.2.5.
[PATCH 03/11] gnu: menu-cache: Update to 1.0.2.

The ones above are functional and can be applied, the ones below are for first 
RFCs:

[PATCH 04/11] gnu: Add lxmenu-data.
[PATCH 05/11] gnu: Add lxde-icon-theme.
[PATCH 06/11] gnu: Add lxde-common.
[PATCH 07/11] gnu: Add lxinput.
[PATCH 08/11] gnu: Add lxsession.
[PATCH 09/11] gnu: Add keybinder.
[PATCH 10/11] gnu: Add lxpanel.
[PATCH 11/11] gnu: Add lxde.

Currently when I reconfigure a system with lxde in the global available 
packages, and I try to log in to LXDE (which is detectecd by SDDM), SDDM for a 
short second shows a black screen and drops me back to the log in screen.

That's the first time I did an meta package for Guix, but I guess I did it 
right?