Introduce a new icon particle. It follows the icon
spec (https://specifications.freedesktop.org/icon-theme-spec/latest/index.html).
Rendering logic is taken from fuzzel (using nanosvg + libpng), while loading
logic is taken from sway. Standard usage is with `use-tag = false` which expands
the provided string template and then loads the string as the icon name. There
are settings to manually override the base paths, themes, etc. The second usage
which is required for tray support is a special icon tag that transfers raw
pixmaps. With `use-tag = true` it first expands the string, and then uses that
output to find an icon pixmap tag. To reduce memory usage, themes are reference
counted so they can be passed down the configuration stack without having to
load them in multiple times.
For programmability, a fallback particle can be specified if no icon/tag is
found `fallback: ...`. And the new icon pixmap tag can be existence checked in
map conditions using `+{tag_name}`.
Future work to be done in follow up diffs:
1. Icon caching. Currently performs an icon lookup on each instantiation & a
render on each refresh.
2. Theme caching. Changing theme directories results in a new "theme collection"
being created resulting in the possibility of duplicated theme loading.
