Pango global font aliases

[Alex Larsson <alexl redhat com>]

Currently some pango backend has some own magic for doing aliases, and 
some doesn't support any, and some of it is handled in the generic code.

I started working on this today, but while discussing the internal pango 
module API with owen we were a bit unsure about exactly what this should 
do.

Here is a list of requirements that i would like to support:

* Allow users to specify that font family "foo" is an alias of a list of 
font families.

* Allow users to specify that certain aliases are visible (i.e. visible in 
the list of fonts).

* Allow the aliases to be shared amongst the Pango backend. This is 
important in for e.g. the canvas that uses the FT2 backend, since it 
should really have the same aliases as the Xft backend used in the rest of 
the app. This could also be important for printing.

* Remove the need for Pango backends to handle subfonts. (Except the X 
core fonts backend which has some special needs.)

* Allow backends to add default aliases. The standard example here would 
be that win32 backend that has default aliases for sans, serif and 
monospace. It would be nice if the standard win32 backend didn't need any 
config file to just work.

Does anyone have other requirements, or think some of these are bad?

/ Alex