ProjectExplorer: Fix comparing priorities

Makes test_modelmanager_extraeditorsupport_uiFiles pass again. Before, a
"CMake Wizard" dialog blocked the auto test due to the wrong
comparisons.

The std::max_element function takes a less predicate. Thus the right way
to find e.g. highest int in a vector is:
std::max_element(..., std::less<>) or
std::min_element(..., std::greater<>)

Both variants are confussing to read. Instead of provinding
Utils::maxElementOr provide a bestElementOr which leads to this code:
bestElementOr(..., std::greater<>).

Change-Id: Ic30f0d742c03170b28227f60d3a5ae00e40fdf5a
Reviewed-by: Daniel Teske <daniel.teske@digia.com>