[pvr] fix sorting for timers window

This fixes the broken sorting in timers window  (a stupid c/p) and changes the item "Add timer..." to stay always on top of the list.

@Jalle19 mind taking a look.
no, if it isn't a folder it is sorted as a normal item.
the same is done for the 'Add source...' button in CGUIMediaWindow
https://github.com/xbmc/xbmc/blob/master/xbmc/windows/GUIMediaWindow.cpp#L807