Fixup qmake lookup

a) Make sure we don't search for qmake executables unnecessarily
b) Make sure we don't scan invalid paths
   (should fix an issue on Windows)
now it doesn't seem to crash, so uncomment
Let cmakemanagertest pass
Fix generation of the cmake build directory by tests
Remove reference to deleted setuphelpers_msvc.cpp.

File was deleted in 1e3643cbfb9e4ab7e2a4a94e179b6385220faf40.
Use #pragma message instead of #warning.

MSVC doesn't have #warning.
Use Q_DECL_NOEXCEPT instead of noexcept.
Explicitly create QList<Variant> when initializing TreeItems.

Otherwise it doesn't compile in MSVC. I don't know exactly why it's needed,
I couldn't create a smaller testcase :/
Don't search the whole filesystem for .qch files
If there's no build directory, ask for it