Better diagnostic message when CMake is not found

Issue

If the user has a valid qmake kit but Qt Creator can't find CMake, opening CMakeLists.txt results in "No valid kits found. Please add a kit in the options or via the maintenance tool of the SDK." This is misleading and confusing (see https://forum.qt.io/topic/98469/another-no-valid-kits-found-error for example)

Suggestion

Instead of "No valid kits found", say "CMake not found. Please add a CMake installation in the options" or similar.