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.

layout/forms/nsImageControlFrame.cpp

@@ -2,6 +2,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
 #include "nsImageFrame.h"
 #include "nsIFormControlFrame.h"
 #include "nsPresContext.h"
@@ -10,6 +11,7 @@
 #include "nsFormControlFrame.h"
 #include "nsLayoutUtils.h"
 #include "mozilla/MouseEvents.h"
+#include "nsIContent.h"
 
 using namespace mozilla;