[pvr] improvements/cleanups for the search window

* removes item 'no search results' and adds a new item 'Search ...'
which do not disappear if the results are shown
* renames ShowSearchResults() to OpenDialogSearch() as we don't show
results, we show the dialog to define the search settings
* refactor method Search() into OnPrepareFileItems()
* removes some needless methods/vars like ActionShowSearch()

@Jalle19 or @opdenkamp mind taking a look
it's possible to select the desired group in the search dialog and IMO limiting to the current selected group if you find similars makes no sense. correct me if I'm wrong :)