Remove stock icons, use icons as resources in the shell

Since one of the previous commits there are no icons being used in libview, there is no longer a need to install the icons as files. Instead, the icons requested by the application can be shipped as resources, which is the recommended way to do it[1]. This is a breaking change for applications that might be using the icons, but setting up icons as resources is very easy, as show-cased in this commit, so should not be an issue.

Closes !302 (closed)

Depends !594 (merged)

[1] https://developer.gnome.org/documentation/tutorials/themed-icons.html