Separate out Icons into stock instead of direct use of pixbufs

[olabiniV2]

We are currently manually setting icons all over the place. That makes it hard to keep track of what icons exist, and also hard to switch them out when we want to do things like change the theme.

We should instead register all our icons with custom stock names, and then refer to them using these stock names from inside the XML definitions. This will allow automatic transition between variants for dark and light, for example.

We might also investigate how to manage icons from inside of CSS, in order to make it possible for end users to customize CSS and icons.

Some things to look at for this might be the icon-name propery and new_from_icon_name for setting images. GtkIconFactory and IconTheme are also possible places of investigation.

[olabiniV2]

We found the solution to this in KeyMirror - it wasn't exactly obvious. The main component for this can be found in https://github.com/digitalautonomy/keymirror/blob/main/gui/icons.go - you need to put the data in a file, and then read it and register it using Gio, and finally add it to GTK as a resource path. The icons are defined in a resource file that gets compiled into this directory: https://github.com/digitalautonomy/keymirror/tree/main/gui/definitions/resources - the source definition for this resource is here: https://github.com/digitalautonomy/keymirror/tree/main/resources