Added several new stats in the creature config file.

Most of them are now used when leveling up.

Also, the actual creature movement speed depending on the terrain is used
instead of its definition base value.

I also added a path() proxy function when called for creature for simpification
purpose.

For now, only the Physical attack/defense are used when fighting.
What's left to add is handling the natural range of the creature and split
physical/magical attack/defense.

Part of #152

Stats added in file.
    DigRate/Level
    ClaimRate/Level
    GroundSpeed/Level
    WaterSpeed/Level
    LavaSpeed/Level
    PhysicalAttack
    PhysicalAtk/Level
    MagicalAttack
    MagicalAtk/Level
    PhysicalDefense
    PhysicalDef/Level
    MagicalDefense
    MagicalDef/Level
Fixed the boulder trap icon position

Fixes #242
Renamed gui/OpenDungeons.layout to some more meaningful name.
Added the Imp (For the Wyvern) and Dragon Bed.
Fixed the layout file name in the CREDITS file.

Thanks to @Akien-mga for pointing out. :)
Added attack configuration range + physical/magical attack actual support.

I also updated a bit the info shown in the developer stats window.

What's left for #152 is the XP Table and the default attack warmup parameter handling.
Fixed the best attack range computation.

Thanks to @hwoarangmy for pointing out. :)
Merge pull request #244 from Bertram25/new_params

New configurable creature stats  + new beds & small fixes