Add support for colors with transparency in palettes and legend.

Modify the color palette reader to support alpha channel values
in each color definition line (ARGB order). If a line has only
three values (RGB order) the color is completely opaque,
providing full compatibility with existing palettes without
alpha values in their colors.

Also use the indexed color image creation workaround
for color bar and raster images.
Introduce pretty vectors and refactor ImageStyle using goodies of enum class.