QuickSearch: features and fixes

See comments on particular commits (included here).

Usage:
```
// now you can also use comma separated values in naming your fields
$grid->addQuickSearch(array('name','phone','email'));
$grid->addQuickSearch('name,phone,email'); // this is the same

// now you can enable cancel button in quicksearch
// by default it's not enabled to be backwards compatible
$grid->addQuickSearch('name,phone,email','QuickSearch',array('show_cancel'=>true));
```