IDE: implement find references of used unit

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42686 4005530d-fff6-0310-9dd1-cebe43e6787f
Tests: more test for PageControl

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42687 4005530d-fff6-0310-9dd1-cebe43e6787f