Unit: serialize hidden= attribute

This allows units hidden with [hide_unit] to remain hidden after reloading a save.

changelog

@@ -95,6 +95,7 @@ Version 1.13.11:
      the variable, which may be either a scalar or a container.
    * A missing [event] name= key will now raise a WML error instead of being
      silently ignored.
+   * Units hidden with [hide_unit] will remain hidden after reloading saves.
  * Miscellaneous and bug fixes:
    * Suggested save file names now use spaces instead of underscores.
    * Fixed crash after canceling add-on download (bug #2203)

src/units/unit.cpp

@@ -385,7 +385,7 @@ unit::unit(const config& cfg, bool use_traits, const vconfig* vcfg)
 	, modification_descriptions_()
 	, anim_comp_(new unit_animation_component(*this))
 	, getsHit_(0)
-	, hidden_(false)
+	, hidden_(cfg["hidden"].to_bool(false))
 	, hp_bar_scaling_(cfg["hp_bar_scaling"].blank() ? type_->hp_bar_scaling() : cfg["hp_bar_scaling"])
 	, xp_bar_scaling_(cfg["xp_bar_scaling"].blank() ? type_->xp_bar_scaling() : cfg["xp_bar_scaling"])
 	, modifications_()
@@ -1492,6 +1492,8 @@ void unit::write(config& cfg) const
 	cfg["max_attacks"] = max_attacks_;
 	cfg["zoc"] = emit_zoc_;
 
+	cfg["hidden"] = hidden_;
+
 	cfg.clear_children("attack");
 	for(attack_ptr i : attacks_) {
 		i->write(cfg.add_child("attack"));