Remove spirit form dependency on WorldType, and add related Lua interfaces.

This commit introduces a second pause mode:
Full game pause (as in menu), and world pause (as in spirit form).
All related checks are no longer done against WT_* constants,
but against the new world pause which is functionally equivalent,
but more flexible. Continuity::worldType is now only used to toggle
world pause correctly, and to apply some related graphical effects.
The world pause can also be controlled via script without actually
switching forms.
Added Lua functions:
+ node_setSpiritFreeze()
+ quad_setPauseLevel()
+ isWorldPaused()
+ setWorldPaused()