Re-register the SpaceShaper for now

Helps with characters like &ApplyFunction; and &InvisibleTimes; not
supported by many fonts.
Port GTK backend to Cairo drawing
Some work into making GTK backend GTK-independent

Ultimately it should depend only on Cairo/Pango.
Don’t pass around unused Configuration instances
Remove unused duplicate files
Fix warning with newer automake
Drop last GDK use in the GTK backend
Rename GTK backend to Cairo
Start phasing out the use of configuration files

The library should not need any configurations files, fonts and sizes
should be set by the clients which may or may not use configuration
files themselves.