Support grouping of cmake targets into sub folders

This patch implements support for grouping targets into common
folders by their FOLDER target property, see [1].

This is great for KDevelop, as we would otherwise show tons of
CTest targets in the root directory. Now, they are all nicely
hidden away behind the CTestDashboardTargets target "folder".

Other projects can also leverage this to group their targets,
if desired.

[1]: https://cmake.org/cmake/help/latest/prop_tgt/FOLDER.html