Add qt_add_shaders convenience option to auto-generate 2-view versions of the shaders

The original idea of everything doing an additional qt_add_shaders() with
VIEW_COUNT 2 GLSL 330,300es HLSL 61 MSL 12 does not scale, e.g. in qtdeclarative there are numerous qt_add_shaders() calls and we'd need to duplicate pretty much all of them to get the multiview (2 view) versions of the shaders.

Therefore, for internal purposes it would be highly beneficial to have a dedicated option, e.g. AUTO_MULTIVIEW, that would automatically do an additional round with the above settings, generating qsb files with some fixed suffix. This way in qtdeclarative and elsehwere we could probably just add this option and be done.