Add support to the slider to format the value

[laeubi]

Currently the slider always show the raw number, but often this is not very suitable for the user e.g. if the Number represents a unit of measure or should be formated with a number format.

This adds a new method NebulaSlider#setLabelFormatProvider that allows to customize the formatting of the raw number.

This then maybe looks like this:

[lcaron]

Good job, thank you Christoph for this PR