UI: Regroup and Reorder source right-click menus

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Visually, the Scale Filtering, Blending Mode & Method, and Deinterlacing
menus are now grouped together, with "Hide in Mixer" being its own group
until a better place for it has been found.
Projector and Screenshot menus have been moved down.

As a related code cleanup, the creation of sourceProjector and
sourceWindow has been moved down and partially inlined.

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