Make init_movit() return a true/false error value.

This allows clients to programmatically enable/disable Movit usage
as needed on very old platforms.
Move everything into “namespace movit”.

This is a pretty hard API break, but it's probably the last big API
break before 1.0, and some of the names (e.g. Effect, Input ResourcePool)
are really so generic that they should not be allowed to pollute the global
namespace.
Fix a typo.