Re-worked the whole keybinding system.

Now handled by scales_ui.keycontrol, and mutable keymaps are now
possible. There is now a distinction between key commands that should
be continuous when held down and those that shouldn't.
Behind the scenes fading and screen changes.

Screens have been improved to a meta-level tool for changing any
aspect of their constituent entities, and the fade function is now
a method of every entity instead of being in a separate module.
Re-organize the taxa system into a class tree.

There is now a definitive namespace hierarchy of taxa, with a
refactored new_taxon function and retaining full inheritance
properties. Five new species added for testing purposes.
Added a new start screen.
Fix default image filter in play mode.
Added multi-part body system. New dragon art.