Easier view mixin declaration for new UI element classes

[CRImier]

Say, there's a new class based off Menu, i.e. MessagesMenu. Let's imagine it needs a single view function redefined. As it stands now, you need to:

• Create a mixin with whatever functions you need redefined
• Create new view class stubs from each existing view from the parent UI element (currently, with BaseListUIElements, there are 3 views, but the count will grow, and custom view plugin support will be added after some time)
• Redefine get_views_dict (or whatever it was called)

I have a better idea that requires less code to be written, especially as people want to use their own views:

• Create a MessagesMenu-specific mixin with whatever functions you need redefined
• In MessagesMenu definition, assign the mixin class to some internal variable

During initialization, say, when get_view_dict is called, it would get the view dict of the parent class, then child classes of all the relevant views would be constructed using the element-specific mixin. Basically, things like these will no longer be necessary.

Problems:

• In a chain of BaseListUIElement -> Menu -> MessagesMenu, mixins of both Menu and MessagesMenu need to be applied
• Not all views need to be auto-created. Say, text-based views with custom overrides, or text-to-speech views (we don't have the latter yet, but already have the former)

[CRImier]

Pushed the initial implementation to devel. It does address the "not all views need mixins" problem, but doesn't address the "mixin chain" problem.

[CRImier]

Another problem I thought of - how do we handle view errors that occur at any stage? What if a custom view's __init__() fails? What if a custom view's refresh() fails? Is it easier to break things with such a setup?