#98 Add possibility to remove color and colorstops from QwtLinearColormap

[Kerim Khemraev]

Hi,

QwtLinearColormap reasonably allows to have multiple colors at the same colorstop. But in case of boundary values (lower/upper boundary) there is no much sense of that.
As there is no evident way to remove colorstops from QwtLinearColormap that makes some inconvenients when creating interactive colormaps.

So I would be glad to see in the future release possibilities to not only addColorstops but also removeColorstops and removeColors from QwtLinearColormap.

Best regards,
Kerim