Bugfix #22030: Animation Editors and Layer Animation

Added new filtering option for animation editors (indicated as the 'ghost' toggle beside the 'select'(-ed only) toggle), which will include objects/bones that aren't visible (i.e. are hidden or on a hidden layer). 

This should make it possible to edit such types of animation, and also prevent flickering as these channels come in/out of view.

source/blender/makesdna/DNA_action_types.h

@@ -550,6 +550,9 @@ typedef enum eDopeSheet_FilterFlag {
 		/* NLA-specific filters */
 	ADS_FILTER_NLA_NOACT		= (1<<25),	/* if the AnimData block has no NLA data, don't include to just show Action-line */
 	
+		/* general filtering 3 */
+	ADS_FILTER_INCL_HIDDEN		= (1<<26),	/* include 'hidden' channels too (i.e. those from hidden Objects/Bones) */
+	
 		/* combination filters (some only used at runtime) */
 	ADS_FILTER_NOOBDATA = (ADS_FILTER_NOCAM|ADS_FILTER_NOMAT|ADS_FILTER_NOLAM|ADS_FILTER_NOCUR|ADS_FILTER_NOPART|ADS_FILTER_NOARM),
 } eDopeSheet_FilterFlag;