Improve custom data support in CiviReport framework

Work done by Dave D on activity custom data can be of great help here. Refer http://svn.civicrm.org/php/trunk/custom_php/CRM/Report/Form/Activity/ActivityFields.php.

Add fixes for following limitations (as quoted by Dave D):
1. Couldn't see any way to give titles to the checkbox groupings in the display columns, so if there's multiple custom groups for different activity types with similar fields you can't tell which is which.
2. Support "IS NULL" for dates and string filters (as done for integer filters).

Fixes and incorporation of Dave D's report must support these requirements (from PTP):

• all 'searchable' activity custom fields are exposed as optional display columns and filters (this includes custom fields USED FOR specific activity types AND those used for 'any' activity type)
• all custom data fields selected for 'display' are included in report's CSV export

We should wind up with one 'improved' activity report which combines our current report with Dave D's report.