Add shortcut button rescale custom range

The rescale to custom range is a very commonly used feature, but it is burried
on the "Color Map Editor Panel". A new button for this feature has been added
in the "Active Variable Controls" toolbar, like the "Rescale to Data Range"
one.

Change-Id: Idd24b88f91ded709af7a3121482c61e8829dd68a
Make discretize color more prominent

The option to "Discretize" the colors with its associated "Number of Table
Values" is regularly used but hard to find. It is now more accessible
in a "Color Discretization" group.

Change-Id: Id05c99a90c887f8ce4f8269c17d2b0def885c779
Merge topic 'rescale_custom_range' into next

cd60f685 Add shortcut button rescale custom range
Merge topic 'discretize_color' into next

0ce9fad4 Make discretize color more prominent