Tab theme configurable

[DLFW]

Text style for tabs can be configured within theme.toml with the dotted keys tab.active and tab.inactive.

The default style is the same as the former hard-coded style.

theme.toml has been restructured and comments have been added to give a short documentation on each style item.

The documentation (theme.toml.md) has been enhanced, and the excerpt of theme.toml with additional comments has been removed from there.

That way it should be easier for devs to keep docs and the default theme.toml in sync, and easier for users when the detail comments can be found in the configuration file itself.

The conversion from AppStyle to tui::style::Style is now provided as a function of AppStyle and a From-implementation for Style to avoid having the same factory code repeating all over.

[DLFW]

Rebased to main, fixed comments, and squashed.

The reason I had used “” is simply that these are the typographically correct quotation marks. However, I understand that this might be confusing and deviate from other places as most people use the ASCII "" quotation. I just have them in my muscle memory. I need to remember to use the "ASCII quotes" while writing prose. :smiling_face_with_tear:

[DLFW]

Just rebased again.

[kamiyaa]

Thanks!