Fixate colors for "UGENE" color scheme (for nucleotides and amino acids)

Background colors of characters of extended alphabets are colored almost randomly, see this function in MsaColorScheme.cpp:

void fillLightColorsColorScheme(QVector<QColor> &colorsPerChar) {

Fixed colors should always be used.

See also: https://ugene.net/wiki/display/UUOUM32/Coloring+Schemes