new feature: apply filter to screen (Ctrl-F)

if one applies a filter, only items that contain this
filter will be displayed. works on all screens.

src/status.h

@@ -34,5 +34,8 @@ void NcmpcppErrorCallback(MPD::Connection *, int, const char *, void *);
 Window &Statusbar();
 void ShowMessage(const char *, ...);
 
+void StatusbarGetStringHelper(const std::wstring &);
+void StatusbarApplyFilterImmediately(const std::wstring &);
+
 #endif