Some polish and bugfix

- Improved auto-balancing.
- Made the first Onslaught mission easier.
- Added damage indicator to gates and walls.
- Made all enemies slightly faster and increased all melee damage speed.
- Increased chance to spawn artifact chests when don't have many artifacts.
- Added some additional sound effects.
- Added resistances to constructs.
- Added 'Random' button for character selection.
- Mixed 'IO' bug that time travel did not actually reduced mission time left + increased time.
- Made 'Priest' slightly weaker.
- Made cheats more accessible via the config file + added more options.