This patch started an attempt to remove nsFrameSelection.h from nsCaret.h
and metastasized into a rather large refactoring patch that removed it
from some other header files as well, and changed nsFrameSelection::HINT
into a global-scope enum with better names. I also converted bools
into CaretAssociationHint in a few places where that was appropriate,
but there are still some more places (GetChildFrameContainingOffset)
where bools need to be converted. I figured this patch was big enough already.
