Configurable classic tibia slots and fixed modern-hand slots

This PR closes #401. It also fixes (or represents real tibia) hand slots where you may only place shield (right hand) and weapon (left hand).
I would prefer that you put slotPosition & SLOTP_TWO_HAND in parentheses to make it easier to understand for people who are unsure about the precedence rules. E.g.:
```cpp
} else if (inventory[CONST_SLOT_RIGHT] && (slotPosition & SLOTP_TWO_HAND)) {
```
(this also applies above, for the right hand slot, and below for the ammo slot)