[smartpl] Add smart playlist support for file modification time

README_SMARTPL.md

@@ -40,6 +40,7 @@ Where valid field-names (with there types) are:
 * year (integer)
 * compilation (integer)
 * time_added (date)
+* time_modified (date)
 * time_played (date)
 
 Valid operators include:

src/SMARTPL.g

@@ -71,6 +71,7 @@ INTTAG		:	'play_count'
 			;
 
 DATETAG		:	'time_added'
+			|	'time_modified'
 			|	'time_played'
 			;