Added and made use of a move ctor in battle_context

(and emplace_back). No reason to use the copy ctor in these cases.

src/actions/attack.hpp

@@ -189,8 +189,10 @@ public:
 	battle_context(const battle_context_unit_stats& att, const battle_context_unit_stats& def);
 
 	battle_context(const battle_context& other);
+	battle_context(battle_context&& other) = default;
 
 	battle_context& operator=(const battle_context& other);
+	battle_context& operator=(battle_context&& other) = default;
 
 	/** This method returns the statistics of the attacker. */
 	const battle_context_unit_stats& get_attacker_stats() const

src/mouse_events.cpp

@@ -955,7 +955,7 @@ int mouse_handler::fill_weapon_choices(
 				best = bc_vector.size();
 			}
 
-			bc_vector.push_back(bc);
+			bc_vector.push_back(std::move(bc));
 		}
 	}
 

src/reports.cpp

@@ -870,8 +870,7 @@ static config unit_weapons(reports::context & rc, const unit *attacker, const ma
 	for (unsigned int i = 0; i < attacker->attacks().size(); i++) {
 		// skip weapons with attack_weight=0
 		if (attacker->attacks()[i].attack_weight() > 0) {
-			battle_context weapon(rc.units(), attacker_pos, defender->get_location(), i, -1, 0.0, nullptr, attacker);
-			weapons.push_back(weapon);
+			weapons.emplace_back(rc.units(), attacker_pos, defender->get_location(), i, -1, 0.0, nullptr, attacker);
 		}
 	}