GuiEvents refactoring + more comments

gui events are now described directly in the GUI class in the gui
elements they belong to.
Implemented monthly transactions and progressbar

Monthly transactions like monthly costs or taxes etc. can now be easily
added to buildings. Also, the sum of monthly transactions is displayed
in the gui.
A new GuiElement has been added: the GuiProgressbar! It can display a
textured progressbar and text. The color of the bar can be changed.
Bugfix: Menu animation

The animation of the background in the main menu didn't use float
interpolation and thus looked quite ugly.
Implemented taxes + GuiNumberbox

Buildings can now pay taxes. A new gui makes it possible to change the
taxes factor. Also, it uses a new GuiElement, the GuiNumberbox. It can
be used to change integer values by clicking on up or down arrow
buttons.
Money panel (#39)

Monthly transactions are now listed in the money panel.
Bugfix: Loading (#45)

Loading didn't work properly when a city was loaded before. Also, the
taxes value wasn't restored correctly.
Merge pull request #46 from ForceField-IndieGames/Benedikt

Monthly transactions + Refactoring