Added city center model
Added building needs feedback (#55)
Added background music
Added user interaction feedback

A text is shown, when a building or a street can't be build, explaining
why that is.
Merge pull request #66 from ForceField-IndieGames/Benedikt

City center model, building needs, feedback, music
Implemented GuiScrollablePanel (#56)
Bugfix: ScrollablePanel [indirect events]

Elements in a did previously not recieve event calls. Also, labels don't
have clipping anymore, since that behavior caused problems with the
ScrollablePanel.
Implemented ScrollBar (#70)
Added Scrollbar to ScrollablePanel
Improved grid graphics
Another grid graphic improvement
Added wave effect to grid
BugFix: Happiness DataView

Buildings that had no happiness value, like streets and the citycenter,
were displayed red in the happiness dataview. They are now displayed
white.
Happiness improvements (#71)
Prepared placeholder object and texture files

+ bugfix (buildings with positiv happinessEffect had a nagative effect
in the corners)
Added happiness effect visualization

while building, a circle shows the happiness effect influence of the
building.
Enhanced city center texture
Added model & texture for medical center (#10)

+ minor gui enhancement. The font under the building buttons in the
building panels is now smaller.
Added model & texture for server center (#10)
Added model & texture for garbage yard (#10)