fix #42: use gtk's text truncation system when possible

[Rayzeq]

Description

This makes labels use gtk's truncation system.
One of the main advantages is that labels can be truncated as required, without having a fixed size.
It also allows pango markup to be truncated (except when show-truncated is false).

Usage

As before

Additional Notes

This PR contains duplicated code, but I didn't find any way to prevent it without removing show-truncated.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing