Use capability rather than category for --sub-filter and --sub-source

That way, only sources show up in the list of sources and filters in
the list of filters.

src/libvlc-module.c

@@ -1678,9 +1678,9 @@ vlc_module_begin ()
     add_integer( "sub-margin", 0, SUB_MARGIN_TEXT,
                  SUB_MARGIN_LONGTEXT, true )
     set_section( N_( "Overlays" ) , NULL )
-    add_module_list_cat( "sub-source", SUBCAT_VIDEO_SUBPIC, NULL,
+    add_module_list( "sub-source", "sub source", NULL,
                      SUB_SOURCE_TEXT, SUB_SOURCE_LONGTEXT, false )
-    add_module_list_cat( "sub-filter", SUBCAT_VIDEO_SUBPIC, NULL,
+    add_module_list( "sub-filter", "sub filter", NULL,
                      SUB_FILTER_TEXT, SUB_FILTER_LONGTEXT, false )
 
 /* Input options */