Utils: Rework CheckableMessageBox

Remove function overloads, thes are hard to read, to use and to extend.
I'd even argue this should be a plain default ctor and a few setters +
exec(), pretty much like Process::start() nowadays.

Move "decider" magic into a structure that can be filled ad-hoc outside
checkablemessagebox.cpp paving the ground for:

...removing  aspect dependency from CheckableMessageBox, Instead, add a
convenience function to BoolAspect.  Arguably, the latter is not needed
and could be done on the user side.

Use pointers instead of mutable references for in-out parameter.
Makes the "specialness" visible on the user side.

Pass ICore::settings() centrally as done elsewhere to reduce line noise
on the user side.

Change-Id: Ibb366353d1ea35401723fd05ce05672617a0a8fd
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/debugger/watchhandler.cpp

@@ -2576,8 +2576,7 @@ void WatchModel::clearWatches()
         ICore::dialogParent(),
         Tr::tr("Remove All Expression Evaluators"),
         Tr::tr("Are you sure you want to remove all expression evaluators?"),
-        ICore::settings(),
-        "RemoveAllWatchers");
+        QString("RemoveAllWatchers"));
     if (ret != QMessageBox::Yes)
         return;