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/git/gitclient.cpp

@@ -1297,8 +1297,7 @@ QStringList GitClient::setupCheckoutArguments(const FilePath &workingDirectory,
             ICore::dialogParent() /*parent*/,
             Tr::tr("Create Local Branch") /*title*/,
             Tr::tr("Would you like to create a local branch?") /*message*/,
-            ICore::settings(),
-            "Git.CreateLocalBranchOnCheckout" /*setting*/,
+            QString("Git.CreateLocalBranchOnCheckout"), /* decider */
             QMessageBox::Yes | QMessageBox::No /*buttons*/,
             QMessageBox::No /*default button*/,
             QMessageBox::No /*button to save*/)