Need a way to disable the select all checkbox

Issue Summary

Hi, There currently isnt a way to optionally specify whether the check all checkbox shows on the view. In my particular circumstance, there are times where views will show items that can be selected but the check all option will never be used. in those circumstances it would be nice to just get rid of the check all option all together.

The attached patch adds a config option in the vbo config area for each view that when checked hides the check all button. this should integrate cleanly into the existing 1.7 release & probably dev too. and since it uses existing behavior by default, shouldnt cause complications for people who dont need it.

incidentally, it might also be nice if there were a way to make the check all option more user-understandable. we have had users complain that the checkbox next to the next table column header doesn't make sense until you see what it does. I'm not sure how to handle that one (and nothing related to it is in this patch, but might be worth thinking about for someone else.

Please take a look at the patch & lemme know if it helps!