Stop using GDK's wrapper on the X11 font functions (GdkFont). All X11

Stop using GDK's wrapper on the X11 font functions (GdkFont). All X11
font operations are now done directly using Xlib calls, and the only
interaction with GDK within the x11font mechanism is to get the X ids
for drawables, GCs and the X display itself.

This should remove an obstacle to porting to GTK3, and also makes the
XFontStruct for loaded fonts more readily available, which I hope will
come in handy for another plan I have in mind.

[originally from svn r9289]