CRM-5902 Activity report -- restrict to current logged in user

Look at http://forum.civicrm.org/index.php/topic,12152.0.html for more details.

Restrict option should be made available similar to "other options" in Event Participant Report (List).

I agree with Deepak's comment that visually it would be better displayed as a checkbox. But I'm not sure it should be separated from the filters, as that could be non-intuitive for people, since this definitely is impacting the resultset (in the event participant report the other options only impact the display – not the query).

I'd rather see the filters tool be improved to allow more flexibility. Specifically:

• add an array option for the filter definitions for "custom clause" or something like that, where if set, the where clause is not auto-created as it cycles through the options, but rather the custom definition is added. Now that I think of it, that would probably be a very easy implementation and provide a high level of flexibility.

• add a new option in CRM_Report_Form for checkbox. functionally it could be built around the multi-select model, but in a use case like this, would allow us to create a single option value

"clause" option is already used in CRM/Report/Form/Contribute/Repeat.php (as fields option). And there is some code written in CRM/Report/Form::whereClause() to consider this property. May be we can experiment and improve that part.

Tested in r30413.