Fix Clang's mismatched tags warning instead of disabling it

Before ed35c62e 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`
works.

Test Plan:
Still compiles, without any extra warnings.