Perf: Optimize ProjectItemDataProvider::itemCount()

A callgrind run showed that ProjectItemDataProvider::itemCount() is called a lot of
times when quick open is opened, but the function itself causes massive re-computation of
known values each time it is invoked.

This patches fixes this, implementing a cache for the total item count
of the data provider.

Testing: After a second callgrind run
ProjectItemDataProvider::itemCount() no longer showed up as an expensive
function.