Added styleable properties for Grid & replaced grid lines with Canvas.

As discucces here: https://github.com/tesis-dynaware/graph-editor/issues/1
I created an intial pull request to add two new styleable properties for the grid spacing and the grid color and replaced the collection of individual lines as nodes with a single canvas element.

The zoomed-in canvas will look blurry. But I think that is another topic which we should fix seperately as the current way of zooming in is not perfect either (the grid lines will get thicker as the whole scene graph is scaled up).