feat: add fill-svg and preserve-aspect-ratio attributes to image widget

[w-lfchen]

Description

• supersedes feat: Add fill attribute to image widget for custom/dynamic svg color #1146, as discussed there
• adds the fill-svg attribute that attempts to replace the color of an svg
• adds the preserve-aspect-ratio attribute to decide whether to keep an images aspect ratio when it is resized

Usage

(image
  :fill-svg "#cba6f7"
  :image-height 200
  :image-width 400
  :preserve-aspect-ratio true
  :path "../github-icon.svg"
)

Showcase

see #1146, @hypernova7 has provided screenshots.

Additional Notes

preserve-aspect-ratio only sets a flag when loading the image and is therefore bound by what gtk allows.
for example, it doesn't work with svg images, but there are no issues with pngs

Checklist

• 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.
  this shouldn't be necessary, since everything is generated automatically, right?
• I used cargo fmt to automatically format all code before committing