Added skill leveling to SkillSet class

The class now remembers a certain base level and
returns a calculated total level in __getitem__.

The level is calculated based on the step given in
the defintion and the EXP per skill.
EXP and level up mechanism for Player
Fixed name of element defence skills
SkillSet provides functions to query skill information
SkillView implementation started

Display the skills, their current levels (base/effective),
the percentage of EXP used for leveling and the EXP remaining
for the next level. A highligh that can be used for selecting
is implemented.

Note that the view does *not* scoll yet!
Balancing: EXP income declines less rapidly

Before, the EXP gained from a fight with equal levels dropped
from 100 to 36 on level up.

Now it will drop
100 -> 60 -> 36 -> 22 -> 13 -> 8 -> 4 -> 0
Forgot to add code to display the SkillView :P