[WIP] Update fonts CSS code to respect CSS standard specs

[Nilvus]

Closes #13084

This change all CSS code for elegant fonts. This needed to also remove some lines on main darktable theme. Lines removed were redundant and unuseful as they all inherit from line 240 with same setting.

I tested changes (on Japan language due to IPAPGothic font set and on Ukrainian one as well as non-Cyrillic language like French) on many parts and didn't see any changes except one:

• all modules titles are less condensed (but remains condensed). I think that new way is better as they were way too condensed and it's more natural regarding fonts.

Now, code use correct way to handle font with CSS, starting from real family, then set light or bold or medium or condensed with font-weight and font-stretch settings.

@victoryforce, @Mark-64 and @dterrahe: your reviews are more than welcomed, especially to test if it's ok on Windows as I can test that part. Be sure to check #13084 issue of course and as far as possible with both Pango release pointed (the one who have the issue and the one who have not). I hope that new code will work with both versions.

@Mark-64: another thing, good if you can check if that PR do not break your fix related to last one on that PR: #7671

[Mark-64]

Unfortunately it doesn't work in Windows.
While I agree to move closer to the Css and Gtk standards, it looks like the current implementation in pango 1.50.12 is quite bugged.
See below some examples using gtk3-demo, but same results in darktable

It kind of works with standard settings

but the "light" versions looks ugly, too thin and syntetic

see Libre Office Writer in comparison

font-stretch statement is not recognized and messes up the rendering

This corresponds to

* {
  color:black;
  background-color: white;
  font-family: "Segoe UI";
  font-weight: normal;
  font-stretch: condensed;
}

Roboto and other user-installed fonts are not recognized:

this corresponds to

* {
  color:black;
  background-color: white;
  font-family: "Roboto";
  font-weight: normal;
}

I believe we should report those issues in pango, I am available. Or @Nilvus you want to do it?

[Nilvus]

Thanks @Mark-64 for your feedback. I understand so that it's worse on Windows by respecting such specs. Have I understood correctly?

It would be good to share to Pango our CSS (darktable-elegant-darker.css file) with the before/after and linking to your detailed comment on that PR. Seems that something miss. Unfortunately, I can't test on Windows to better work/understand the issue. On Linux, I do not have such huge differences. I so prefer that you precise your feedback and link to my work. If needed, I can complete or answer to Pango issue (and I follow it already).

[Nilvus]

Nothing moves on Pango and Cairo issues. So closing. Could be redone and updated regarding master if this changed.