GUI serialization

[kiith-sa]

GUI should be serializable to/from yaml.

There should be a default set of values in a mod directory for e.g. GUIButton, and widget-specific settings can override that.

Of course, serialization can't be without limits, e.g. signals still need to connect, but we should at least be able to
e.g. choose implementation of a menu. Still, consider even full serialization with signals specified in yaml.
That might work if we have some sort of dynamic signal object, any number of which could be added to a GUI element.
Might be possible to do that elegantly in D2.

Separate from GUI layout, there should be skins/themes. One skin per entire GUI. The skin could specify:
border, background colors, font size, possibly Z index like HTML/CSS, maybe also element dimensions, not sure about that as it might end up with one skin declaration per element.

The skin would be in CSS-like format, using #id' s and .class'es to identify elements.

There should be a GUIStyle class separate from GUIElement, dedicated to this. Each GUI element would have a link
to its style and use that for drawing/updating/whatever.

Also, we need an option to reset and reload entire GUI subsystem at runtime (at least in the main menu) - this would
be immensely helpful for fast modding/testing.

Dependencies: #9