[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
If there's a group selection thingy on the ui now, sure. Only getting github's email now here on my phone, can't really review this ;)

Limiting to the currently selected group prevents this function from showing matches from channels that I really don't want to see the matches from (eg from some asian channel without subs that I can't understand a single word from)

> Op 1 aug. 2014 om 20:04 heeft Sascha Woo <notifications@github.com> het volgende geschreven:
> 
> In xbmc/pvr/windows/GUIWindowPVRSearch.cpp:
> 
> > @@ -142,34 +150,22 @@ void CGUIWindowPVRSearch::Search(void)
> >      if (dlgProgress)
> >      {
> >        dlgProgress->SetHeading(194);
> > -      dlgProgress->SetLine(0, m_searchfilter.m_strSearchTerm);
> > -      dlgProgress->SetLine(1, "");
> > -      dlgProgress->SetLine(2, "");
> > +      dlgProgress->SetText(CVariant(m_searchfilter.m_strSearchTerm));
> >        dlgProgress->StartModal();
> >        dlgProgress->Progress();
> >      }
> >  
> >      // TODO get this from the selected channel group
> 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 :)
> 
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