New scenario element "AbsorbingArea" is not handled in:
- AbsorbingAreaController (this class is missing completely at the moment).
- MigrationAssistant (must add node '"absorbingAreas" : [ ]' to topography tree).
- DefaultConfig (to be able to change color).
- TikzGenerator (must use "topography.getAbsorbingAreas()" to iterate over the new scenario element).

Extracted filename prefix for snapshots to "messages.properties" and "messages_de_DE.properties" respectively.

Fix #160: In "TikzGenerator.java", use "String.format(Locale.US, ..." to force a dot as decimal separator.

In "TopographyWindow" and "Online/PostVisualizationWindows", replaced "About" button by "Setting" button.

implementation of the gui elements and actions to merge overlapping obstacles. The algorithm sometimes still fail and the undo/redo actions are not jet implemented.

In "PostvisualizationWindow.java", added button to generate TikZ image.

Action "ActionGenerateTikZ" is a copy of "ActionGenerateSVG".