automoc: add a global AUTOMOC_TARGETS_FOLDER property

This has been requested, along with a patch, by Shaun Williams
 in http://public.kitware.com/Bug/view.php?id=13788 .
It adds a global property AUTOMOC_TARGETS_FOLDER, which can
be used to group the automoc targets together in a folder e.g.
in Visual Studio.

Alex
Merge topic 'AddAutomocTargetFolder' into next

ba0e06f automoc: add a global AUTOMOC_TARGETS_FOLDER property