common: Provide Singleton mixin for handling pseudo singletons

common: Provide Singleton mixin for handling pseudo singletons

We have several singleton and pseudo-singleton classes in Quassel.
A pseudo singleton can be constructed and destroyed in a controlled
manner, but can be accessed through a static instance pointer while
it is alive. As an example, QCoreApplication behaves like this.

In order to avoid duplication and unify the behavior of our singletons
in the future, provide a mixin. Classes can inherit from this and gain
an instance() method, as well as protection against multiple
instantiation and access outside of the object's lifetime.