RFC: Introduce QtQuick effect API

In order to provide a simple way to write QtQuick based fullscreen effects, it's worth adding a QtQuick effect API in libkwineffects.

The Overview effect can be used as the seed. Potential api may look as follows

class KWINEFFECTS_EXPORT QuickScreenView : public OffscreenQuickView
{
    stuff
};

class KWINEFFECTS_EXPORT QuickSceneEffect : public Effect
{
    Q_OBJECT

public:
    explicit QuickSceneEffect(QObject *parent = nullptr);

    bool isRunning() const;
    void setRunning(bool running);

    bool acceptsPointerEvents() const;
    void setAcceptsPointerEvents(bool accepts);

    bool acceptsKeyboardEvents() const;
    void setAcceptsKeyboardEvents(bool accepts);

    bool acceptsTouchEvents() const;
    void setAccepsTouchEvents(bool accepts);

    QList<QuickScreenView *> views() const;
    QuickScreenView *viewAt(const QPoint &pos) const;

protected:
    virtual QuickScreenView *createView(EffectScreen *screen) = 0;
};

Another advantage of this approach is that the QuickEffect could have internal caching logic to avoid re-parsing qml files when the overview effect is toggled.

Class name candidates

• QuickEffect
• QuickSceneEffect
• QuickViewEffect
• QuickScreenEffect
• QuickWorkspaceEffect
• QuickFullscreenEffect
• other?