A collection of multiple bugs in gfxbuilder

As I stumbled over multiple bugs in gfxbuilder, I will summarize what I did in this job:

• Added removeNode() to AbstractDrawableContainer
• Fixed a missing break in AbstractDrawableContainer EventHandling
• Fixed a bug so now borderWidth = 0 means the border (or the whole stroke) is invisible.
• Removed fixed 100px gradient distance in AbstractLinearGradientPaintNode
• Fixed a bug that made using color() inside of gradientPaint impossible
• Fixed a bug in PolylineNode (changed type of points from double to List<Double>)
• Fixed GRIFFON-154:
  An Area of a Line is always empty as its Shape is unclosed and not closable (see javadoc of Area)
  I am temporarily adding an invisible RectangleNode around each 'empty' outline like a line if is used in a group()
• Introduced basic tools for testing images
• Introduced test cases for bugs