#16 renamed ValidationJTextField.isValid() to ValidationJTextField.isContentValid()
#16 moved the calculation for the new pixel sizes and DPI to a inner class of ExportMapToFileDialog.
#16 renamed the variabel DPI to dpi
#16 changed the validation pattern of the width and height textfield
#16 when the user changes the pixel size, the dpi changes too
#16 when the user changes the dpi, the pixel size changes too
#16 implented other way to calculate dpi. The old one had the problem that if dpi becomes zero, it could not be recalculated anymore.
#16 override of equals() of ImageDownload and WorldFileDownload
#16 minor changes in the appearance of the GUI
#16 draw a white background before saving image