Moved some extern declarations from TaigaInit.hpp to Taiga.hpp

The variables affected are taiga_display, taiga_events,
taiga_ticktimer, taiga_frametimer, and taiga_quitting.
Add TaigaActorList::paused field
Remove extern declaration of 'actors' in TaigaActorList.hpp
Add TaigaActorList::clear()
Protect against recursive calls to TaigaActorList::tick()
Protect against clearing actors from a ticking TaigaActorList

It is dangerous because it can cause the iterator to go out of sync
with the list, even if you're iterating with an integer.