Make QQEM to match other Qt modules

QQEM has been structured to be a stand-alone application. If it will be part of Qt modules (QTBUG-108160), adjust it to match other modules.

• Sources could be moved into "tools/qqem" like other tools.
• Adjust CMake files to be similar with other Qt internal tools. For example instead of "qt_add_executable" it should probably use either "qt_internal_add_tool" or "qt_internal_add_app".
• Application binary should be installed into bin directory, e.g. "C:\qt6-install\bin\qqem.exe".
• Where would data (nodes, QML helpers, images, qsb shaders) be installed? Under "C:\qt6-install\qml\QtQuickEffectMaker"?