#3 Connection label support

[Doug]

The attached file is a replacement for the PaintLabel()
function in NetronGraphLib/Connection.cs. The
additional functionality it provides:

1) Draws label text for EVERY type of connection
LinePath, including Bezier and custom (Thermo, etc).

2) Rotates the text on all but Rectangular LinePaths.

3) On Rectangular, the positioning logic is changed to
center the text on the longest line segment.

4) The background rectangle drawing has been disabled
(commented out), as I find it visually distracting.
Perhaps showing the background rectangle could be
controlled by a property?

[Doug]

replacement for PaintLabel() function in Connection.cs