Animation editor: Add D, Shift+D, Ctrl+Shift+D key combos to flip bone rotation.
Fix some SceneEditor crashes/quirks and remove some dead code in the process.
Map element code refactoring. Allow other entities to influence map elements (EFX_WAVY).

Reduce Element memory; as most elements do not have any effect set,
this saves more than 50 bytes per element.
Script interface:

- Add entity_doElementInteraction()
- Add avatar_setElementEffectMult()
- Change entity_isUnderWater(): Can now supply an override position.