QuickSearch: features & fixes

1) Implementation of "cancel" button (backwards compatible)
Can use it if call QuickSearch like this `$grid->addQuickSearch(array('name'),'QuickSearch',array('show_cancel'=>true));`
2) Buttons moved inside ButtonSet. This allows nicer CSS styling.
3) You can now also pass comma-separated values (string) of fieldnames when initialize QS.
Like this: `$grid->addQuickSearch('name,email,phone');`