Fix Clang's mismatched tags warning instead of disabling it
Before ed35c62e9f62 added an inhibition to CMakeLists.txt, Clang
showed the following warning:
class 'QuickEditorPrivate' was previously declared as a struct [-Wmismatched-tags] Q_DECLARE_PRIVATE(QuickEditor); ^
This can be fixed properly by making QuickEditorPrivate a class
instead of a struct, which matches better with how Q_DECLARE_PRIVATE
Still compiles, without any extra warnings.