runnable edit: use HSLColor to replace Color
fixed Issue #455: Brightness slider is buggy at extreme ends

1. cause:
ColorPane used RGB color (Class System.Drawing.Color) to store the selected color information.
When express its saturation & brightness value, it need to be converted to HSL (Hue-Saturation-Brightness) color.

The issue is, the stored RGB color cannot be converted to HSL color without losing information.
e.g. the RGB color generated from HSL color with its brightness to be none or full, will actually lose the color info (Hue value).
Thus it causes the bug: when move brightness to the extreme ends, it loses color info; when drag it back, it then uses the default color info (normally red color).

2. sol:
use HSLColor to store the selected color information. Thus all the required info are there, and no issue when converted to RGB color.
make all bindings work
fixed a bug that cause theme color to be black aft move trackbar
make it runnable, and remove preventable exception
beautify the colorPane