Create a documentation (and example?) for working with opacity masks

Effect composer's opacity mask effect now supports also using items as masks. However, the proper use of item masks is not necessarily very intuitive, so it needs to be properly documented, and a simple example wouldn't hurt, either.

In a nutshell:

• Effect composer's opacity mask effect has "Source" property. On effect composer you can only set this to an image, but when the effect is added to a scene, you can also set another item as source item for the mask instead of an image.
• The source item is scaled to the effected item, which may not be desirable. The proper way to address this is to have an invisible Item with same size as the effected item as source item, and have the actual masking item as a child of the source item. If you have the source item anchored to the effected item, then the actual masked part will correspond exactly to the masking item.

See QDS-13501 for additional discussion on the matter.