Update characters_editor implementation, assets and documentation

locale/ui_en.properties

@@ -20,3 +20,10 @@ tools/characters_editor/behaviour = Behaviour
 tools/characters_editor/behaviour_tip = How the character acts in battle when the game controls it; base — the usual
 tools/characters_editor/actions = Actions
 tools/characters_editor/actions_tip = The character's battle actions from the loaded mods
+tools/characters_editor/section_party = Party
+tools/characters_editor/leader_yes = Party leader: the player walks on RPG maps with this character's sheet
+tools/characters_editor/leader_no = Not the party leader: on RPG maps the player looks like the leader
+tools/characters_editor/section_computed = Computed battle numbers
+tools/characters_editor/computed_hint = With Stats, health, attack and defence come from the stats: edit the stats, not these fields.
+tools/characters_editor/computed_none = No Stats: the battle numbers are the fields above.
+tools/characters_editor/computed_row = {0}: {1} (battle {2})