MusicWheel::JumpToNext/PrevGroup, functions tweaked (thanks juanelote)

Metric added, [MusicWheel]::HideSections, will restrict showing one group at time, it you want to see the other groups use the next/prev group code, defaulted to "false"
Also group changing is allowed via gamebuttons in case of those who wants to take advantage of the codedetector too, "false" as default
SongManager::GetSongGroupByIndex, gets groups by index (helpful in Jump to group functions, thanks to juanelote for this too).

src/SongManager.cpp

@@ -706,6 +706,11 @@ int SongManager::GetNumCourseGroups() const
 	return m_mapCourseGroupToInfo.size();
 }
 
+RString SongManager::GetSongGroupByIndex(unsigned index)
+{
+	return m_sSongGroupNames[index];
+}
+
 RString SongManager::ShortenGroupName( RString sLongGroupName )
 {
 	static TitleSubst tsub("Groups");