The current icon style is somewhat all over the place, with some
icons having thin lines, some others having thicker lines; some
filling the pixel grid in center, others in edges; some trying to
reduce subpixel positioning, some completely unaware of it.
Rework these icons to have a consistent style, with elongated
lines, rounded strokes, and proper center alignment.
The "no_sources.svg" asset is not covered by this work because this
empty state will be worked on in a separate commit.
Further changes will adjust specific UI elements to better match
the icon style.
These icons are derived from the GNOME icon library. I've changed
the fill colour to #fefefe for Dark theme icons, and #000000 for
embedded icons. If further icons are needed, they can be found at
https://teams.pages.gitlab.gnome.org/Design/icon-development-kit-www/
