3.x -> 2.3.x compatibility for minimum-distance

This makes minimum-distance behave the same as it did in version 2.3.x. The option is replaced by text-margin and repeat-distance in version 3.x. An exception is thrown if both minimum-distance and one of text-margin or repeat-distance are defined for the same symbolizer. This will allow backward compatiblity with old styles until users can upgrade to the new options.

I also modified one of the visual tests, repeat-labels-1, to test the functionality of minimum-distance, while repeat-labels-[2-4] test text-margin and repeat-distance.