#13 Scale header height based on DPI

[David Wilhelm]

Currently, header height is hard coded in TreeViewAdv.cs, which makes the header too short on high DPI displays.

The attached patches get the vertical DPI from a dummy object and scale the header accordingly.

[David Wilhelm]

Attaching an updated second patch to replace, for not just the header height, but also for NodeCheckBox and the NodePlusMinus boxes.