Add an option to hide qx.core.MObjectId methods

[cboulanger]

Since the qx.core.MObjectId API methods are attached to qx.core.Object, they show up everywhere in the API viewer, even though in the rarest of cases people will be looking for the documentation of these methods in the case of the current class. We should maybe add an option that hide them by default (maybe except in qx.core.Object). Most elegantly, but also quite complex in implementation would be to be able to have a dropdown that allowed to hide individual mixins that are present in the class that is currently being displayed.

[cboulanger]

Maybe what is really the issue here is that I want to be able to see the mixins that a particular class includes explicitly (such as qx.core.Object including qx.core.MObjectid) but that showing Mixins from parents (and higher in the class hierarchy) should be optional. Plus maybe mixins included by Abstract classes should show up in the implementing classes.