IMO even better would be a "select all" button in the task manager:
that could be used in combo with "delete selected" to do what you
suggest, but could also be used with "stop selected" and "restart
selected".

It would also make "delete all" require two buttonpresses, making it
less likely to hit by accident than a "delete all" button.

Does this sound good?

I had originally imagined a button next to the "x" cancel last task
button on the button bar that would cancel all tasks, rather than
having to click on the task manager button, then selecting all tasks,
then deleting those tasks.

But I believe your idea is better with respect to keeping things
organized, because it will put all of the tasks manager operations in
the task manager and it would prevent accidental deletions like you said.

My idea was a little more about convienience. One click to cancel all
tasks.

Right click menu on the little task button at the botom of the main 
window to give stop all tasks/restart all tasks??? 

No I am talking about the button bar at the top of the main window.

Per the call-to-arms message :), I'd like to have this feature and could use it.  I don't really care much one way or the other which form it takes, just having that capability (stop all or select all with ability to do actions on selection).

Either one works for me, too.  I'm clumsy, and have cats. Both have resulted in lots of unwanted items in the task list quite a few times :)

This problem has been fixed in the development version. The fix will be available in the next major software release. Thank you for your bug report.