Factored some part of the sound manager to make it more reusable.

Also added WALK, HURT, and DYING possible creature sounds.
Upgraded music handling + some bugfix.

- Added support to play a music file using its name.
- Added fade in/out support + cross-fade support.
- Added the map info parameters and used the level music parameter.
- The music now cross-fades from the menu music to the in-game one
  and vice-versa.

I'll have a look at supporting battle music in another commit.
Dropped the mana creature parameter.

It will be replaced by skills + cooldowns.
Added menu click sounds.

Moved the former click sound as trap build sound and added a click sound from Valyria Tear for now.
The former click sound was not very well fitting.
Merge pull request #125 from Bertram25/development

Several cleanups and improvements