Fix 16-bit to 8-bit color conversion

We need to divide by 256 not 255 otherwise some colors will be outside of the
valid range.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
channel_color_settings_cb(): Free color chooser dialog

Call gtk_widget_destroy() to free the color chooser dialog and not leak any
memory.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
channel_color_settings_cb(): Set current color as active color

Set the color chooser's active color to the current color of the channel before
showing the dialog.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
oscplot: Use lighter colors for the default colors

The current default have a rather bad contrast against the black background. Use
lighter colors instead. And while we are at it also add some more colors.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>