Remove some unused variables from struct Monster.
New function, bool Game::is_empty(), returns true if the target tile has move_cost > 0 and no Entity.
Add variables to Monster to track number of summons used, and the UID of their parent (if any) (so we can reduce the parent's summons counter when we die).
Fix capitalization, and make all effect() functions use a Monster pointer rather than an Entity pointer.  We don't need to generalize here, since only Monsters will use these; and the code will be cleaner if we use Monster directly.
Rename "owner_name" to "owner" in Dice::load_data() (for consistency with other load_data() functions).
Fix a mistakenly regexed commet.
Add a TODO comment re: get_name_to_player() for friendly monsters.
Add a comment explaining Monster_ability::effect() a bit.
Finished Monster_ability_summon, Monster_ability_signal and Monster_ability_terrain.  Two more (until I add some more) to go!