Figure out the different between views->exposed_data and views->exposed_input

After digging through the code (with the help of grep), I believe that the existence of $view->exposed_data is a bug. $view->exposed_input is used in fewer places, but it does have an intelligent getter and setter. They sometimes hold the same value - when they diverge, it's only because ->exposed_data was udated, while ->exposed_input was not. But we already have raw_exposed_data for holding the original values from the form, if they're needed for something.

Some quick testing shows that replacing all references to exposed_data to exposed_input (and preferring to use $view->set_exposed_input) has no visible side effects. I'll create a patch if we decide to go in that direction.

I'd rather have set_exposed_input set $view->exposed_input, since we can't change a public function name randomly.

I have no memory of why these are different, so it may have just been a side effect of being sleep deprived when I wrote the exposed filter code.

Maybe don't worry about it for D7, and fix it in core if still applicable.