[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
Is it necessary to override a method just to call the parent in C++?