Draft: Restyle

Hello everyone! This is one of my attempts at learning some C while contributing to GNOME simultaneously!

This is a very early draft at porting GNOME System Monitor to the new restyle designs by Allan Day.

I'm a beginner in C and GNOME System Monitor, so this code may have bugs. It's also a draft now, so many things may be broken right now, and don't mind the leftovers and inadequate commit messages and history. I will fix them later. The changes also currently only relate to the Resources tab. I'm just putting this code here just in case I won't be able to work on it further. Thanks!

Screenshot:

There are also some unanswered design questions:

1. Where did the colour changers go in the mockups?
2. I'm confused if the CPU indicators are to be merged. If not, how would many CPUs be indicated (for example, 64 cores.)

(cc @aday)