qgis: Modernize formula and fix a few bugs.

- Remove subformulae bison, qwt
  (they are separate deps now and they seem to work)
- Fix issue that a brewed Python framework was not found by cmake
- Move PyQt availablilty-check to a Requirement
- Remove custom accessor functions "grass?". We have now build.with?
- Expat is a dependency qgis searches for (and uses)
- grass and postgis support is :optional (→ auto generated --with-grass)
- Install qgis into Frameworks and not lib as it provides Frameworks.
- Fixes #23 by adding -F to the Qt Framework