Set window background

The default background color of windows is usually white, which doesn't
necessarily look right. Set the background of the window to
SystemColors.ControlBrushKey instead.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Reset port text box color when parsing succeeds

When we successfully parse the port number we remove the background. This
will not reset the background to the default, it will make it transparent.
This make the control look like it has the same color as the window, which
will sometimes make the control hard to read.

Clear the value of the background property when parsing succeeds. Doing
that will give it the current default color which is usually a good
choice.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>