Use gresources instead of manually including data into rust sources

This modifies the project to use the GResource framework for including resources into the application, which replaces the previous method of using rust macros to include those directly into the source code.

The main improvement is that GtkApplication will automatically load resources under some well-known paths.

This requires a new config.rs file configured by meson, as well as installing the resource bundle into the system/container.

This means that a build using only cargo is no longer supported, and also unfortunately that running meson builds without installing is no longer supported, either.