Multiple levels and stone trolls

The game now has three levels. This means you start at depth 3, and every time you walk onto a set of stairs, you go up one level in the dungeon. Going up from depth one means you have reached the surface, completing the game at this stage in development. I plan to add a system that assigns monsters and items to levels automatically, currently this is still configured manually.

I added a new monster, the stone troll. It will only move if angry. It will become angry if it is annoyed three times, either by your proximity or by an attack. The 'annoyed' count will go down again over time (after 8 turns). The anger will also wear off (12 turns). This is not yet visible to the player. In general, effects that are active on monsters, such as 'annoyed' and 'angry', are not yet displayed.

The stone troll is an example of what I want to add more of to the game: monsters with particular behaviours, movement rules and actions, a bit like chess pieces.

Other aditions in this update include:

- equip action will swap item with already equipped item
- killed monster drops all equipment
- troll cannot open doors but can destroy doors
- chasing monster can use last seen player location
- chasing monster will not always (re)open door immediately
- added monster behaviour to equip best items from bag or ground
- added monster behaviour to move to best items in view
- attack action message will show target's remaing hp
- changed tile graphics