feat: add improve for Epd option, add active-color-label

[flxzt]

This adds the following:

• A label that displays the current active color as a string in the format \<brightness>,\<saturation>,\<hue> with easy to understand strings.
• An "optimize for E-Paper displays" option in order to toggle the visibility of this label. In the future more modifications to the app behaviour and UI additions can be enabled through it

Still a draft, because rnote-compose now needs to be translated with gettext as well. I'll see how to best integrate or refactor it

addresses #658

[flxzt]

@Doublonmousse if you want/have enough time, please review!

[flxzt]

Good points, thanks.

I think I misunderstood the value field in the hsv colorspace while implementing this. I now fixed and improved the logic by:

• including checks for value and saturation to detect white and black
• accounting for float inaccuracies by using approx::rel_eq with f32's epsilon for min/max values. It's relevant because the picked colors come from gdk, which only has f32 precision.
• hand-tweaked the ranges for colors. This is the part which will probably need further testing, because I don't have a calibrated screen. But at least the Gnome colors in the colorpicker are now always in the right category. Ideally there would a definitive (scientific) source for defining hue values as strings, but didn't find one yet.
• added alpha strings transparent, translucent, slightly-translucent and `` (empty string == opaque)