Enables scaling for High DPI screens

[sphh]

My machine has a High DPI screen and prior to these changes the fonts and nodes are barely readable. These changes scale the fonts, the icons and the canvas according to the dpi settings of the screen.

They also introduce preference settings:

• to enable automatic scaling of the canvas.
• to set the initial zoom level either manually (autoscaling switched off) or as a ratio of the automatic scaling.