Optimize: Use a sorted QList<ProjectFilter> instead of a QMap.

The QMap approach has an overhead of at least 8 bytes per shared
QString key. The benchmarks also show that this version is faster
than the QMap version by approx. 40% according to
QuickOpenBench::benchProjectFileFilter_addRemoveProject.

Furthermore this will allow us to overwrite the sorting order to
take temporary files generated in out-of-source build directores
into account.