Server config redesign

This pull request splits the QgsConfigParser/QgsProjectParser classes into subclasses for WMS/WCS/WFS support and two classes for methods which are of general use (QgsServerProjectParser, QgsConfigParserUtils).
Another change: each server has a method executeRequest() which takes the 'if request == ...' code out of the main file.

I've done regression tests for WMS/SLD, WFS, WCS. Could you do some testing too?