Update characters_editor implementation, assets and documentation

locale/s6_en.properties

@@ -0,0 +1,5 @@
+tools/characters_editor/join_check = Joins the party when talked to
+tools/characters_editor/join_tip = A companion: once it joins, the character walks behind the hero. Writes or removes this mod's join scenario at once.
+tools/characters_editor/join_hint = Place this character on a map from the Characters tab of the RPG map editor. Talking to it there makes it join the party; the placed character then leaves the map and walks with the hero. The leader never joins.
+tools/characters_editor/join_on = Joins the party: the join scenario of {0} is written into this mod.
+tools/characters_editor/join_off = Joins the party: the join scenario of {0} is removed.