v20200916

• Add Meson build system
• Drop pre-generated CSS files
• Add sassc dependency
• Drop bc dependency
• Improve source file structure
• gtk: Add initial support for libhandy
• gnome-shell: Add support for version 3.38
• gnome-shell: Update calendar style to match Material Design guideline
• metacity/unity/xfwm: Update title bars to match the last GTK changes
• chrome theme: Improve and simplify
• Other various improvements and fixes