RFC: Turn Playing Time function into plugin

Since this function already requires hitting the
disk, it may make sense to turn it into a plugin.

A minor advantage (apart from cleaning up onplay.c
and saving RAM) is that you can now access the menu
not just from the WPS context menu, but also from
the Shortcuts Menu or using the WPS plugin shortcut.

On the other hand,  TSR plugins would have to be
terminated when Playing Time is launched, as is
already the case for other plugins such as PictureFlow.

Change-Id: Iea85229486887463ffc52f05e33e2340437f69a4

apps/plugins/SOURCES

@@ -18,6 +18,7 @@ logo.c
 lrcplayer.c
 mosaique.c
 main_menu_config.c
+playing_time.c
 properties.c
 random_folder_advance_config.c
 rb_info.c