Finally set a window icon on X11

Since the creation of minetest, it had no window icon on X11.
Now we have one.

The misc/minetest-xorg-icon-128.png file is a rendering of the
misc/minetest.svg file with inkscape, created with something like:

inkscape -z -e misc/minetest-xorg-icon-128.png -w 128 -h 128 misc/minetest.svg