effects/cube: New effect

This change implements the desktop cube effect using QtQuick 3D. It's a
much simpler version of the old desktop cube effect. You can rotate the
cube and click LMB to switch to the currently viewed virtual desktop.

Keyboard navigation is simple too:

- `Escape` - quit the effect
- `Left` and `Right` arrow keys - rotate the cube left or right, respectively
- `Enter`/`Space`/`Return` - switch to the currently viewed desktop

Mouse navigation:

- LMB click - switch to the currently viewed desktop
- Press LMB and drag - rotate the cube
- Wheel up and down - move the cube farther or closer, respectively

The cube effect reserves `Meta+C` keyboard shortcut. You need to have at
least three virtual desktops in order to activate the effect.

BUG: 438883
latest merge request