Gradient Stop Sorting by Value

Adds right click context menu to gradient editor widget with the ability
to sort gradients by value. 'Sort by Value' simply sorts the values onto
the gradient by the value of the color and thus is distributed
appropriately. 'Sort by Value (Even Distribution)' sorts the stops by
value but instead of placing them by color value onto the gradient, it
evenly distributes the colors across the entire gradient space. Both of
these values can be flipped using the 'reverse' button or the reverse
option in the right click menu.