Bug 449848 - Startup: initialize fontconfig in the background ...

... to avoid long pause on start

On non-Linux operating systems the fontconfig cache is often not
initialized by default. The first time GIMP was launched, this led
to a non-responding application, confusing many users.

The initialization of fontconfig has now been moved to a separate
thread. The main thread will wait for this fontconfig thread to
complete, regularly pulsing the UI.

This patch was partly based on an earlier patch by Tor Lillqvist.