Doc: Fix documentation issues for Qt Quick Dialogs

 - Document clickedButton property for Dialog, MessageDialog
 - Update Dialog::accepted() and rejected() documentation
 - Remove duplicate documentation for Dialog::visible property
 - Update description of modality property for ColorDialog,
   FontDialog
 - Fix broken links

Change-Id: I5b73ea5cbccf1ad0203351730197336e1c6e1b98
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>