new loplugin: VCL widget reference checker

First stage of new VCL widget reference checker

Change-Id: I63a2108a26b3c0e0a896d13672b1daa6f8e60b3a
Reviewed-on: https://gerrit.libreoffice.org/10427
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>