Add TextMenuExt.FloatSlider

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I think having an IntSlider, but for floats, for options like percentages or multipliers (something with like 2 digits of precision) would be useful.
It behaves almost identical to IntSlider, but also allows the user to input a number directly by going to OuiNumberInput.

At the moment only the class exists and can be used via something like this:

public float MySetting { get; set; }

public void CreateMySettingEntry(TextMenu menu, bool inGame) {
  menu.Add(
    new TextMenuExt.FloatSlider(Dialog.Clean("..."), 0, 100, MySetting, 1)
      .Change(value => MySetting = value)
  );
}

I'm planning on adding support for automatically creating FloatSliders if a property type is float, as well as customizing its parameters.

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Closing this PR due to a lack of activity. Furthermore, I don't think there's a lot of need for a FloatSlider.