Bug 1048752. Part 16: Move nsFrameSelection::HINT to CaretAssociationHint.h. r=tn

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.

dom/plugins/base/nsPluginInstanceOwner.cpp

@@ -55,6 +55,7 @@ using mozilla::DefaultXDisplay;
 #include "mozilla/MouseEvents.h"
 #include "mozilla/TextEvents.h"
 #include "mozilla/dom/HTMLObjectElementBinding.h"
+#include "nsFrameSelection.h"
 
 #include "nsContentCID.h"
 #include "nsWidgetsCID.h"