Recording activities in a case that are NOT with the client.

Rationale
========
Some case activities (meetings, phone calls, etc.) are ABOUT the case but the 'target' contact (WITH...) is NOT the client. Currently there is no way to record these in a case.

Implementation
============
1. On Edit case activity form (civicrm/case/activity), add link to the right of the read-only Client Name value: 'With other contact(s)"
2. When clicked, replace Client Name label and value with the target_contact_id field label and widget we use in non-case activity form ('With Contact') - so user can select one or more contacts from the DB.
3. Post-process saves target contact id values for this activity
4. Add 'With' column (after 'Type') to case activities selector (Manage Case, Find Cases and Dashboard drilldowns).
5. When editing or viewing case activities, we need to detect whether target contact id = client contact id or not and display proper label (and include editable 'target_contact_id' field with defaults set) when target != client.