qbs build: Introduce item QtcProduct.

QtcAutotest, QtcLibrary, QtcPlugin and QtcTool share quite a bit of
code, which is now factored out into this new item from which they all
derive.

Change-Id: Ic947a19e121f37d34968b29c94dd02648da1cd5b
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>