Mana bars added

The bars system has been refactored. It's now possible to easily extend
it to create different bars i.e. stamina, rage, energy, etc..
The interface anchor has been removed and replaced with a proper control
node to hold the bars. Folder structure has also been cleaned.

Note that the newly added manabar will not show as of now since the
characters do not have mana at level 1 (not sure why). They were
configured to only appear if the battler has more than 0 of max mana.

Closes #31