Categorised styles for regions layers - 3SP

[bariela]

Is your feature request related to a problem? Please describe.
Layers can have categorised colour only in case of predefined set of accepted values. We would like to define numerical intervals as categories

Describe the solution you'd like
Define new colour styles for regions layers, with categories with min max values for given value
interval should have min value included, but max value not
in case of only one value, take everything less than only max or everything more than only min value

Documentation and tutorials should be updated accordingly.