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

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Available options: