feat(Detection): DE and GTK2/3 Detection

[KittyKatt]

Needed to mirror screenFetch functionality.

Detect current running DE (or none) and current GTK2/3 theme.

Needs:

• DE detection
• No DE logic for GTK2/3
• macOS/Windows logic
• Port over and improve previous screenFetch theme detection logic
• GTK2/3/Font improvements and optimizations
• Shorthand logic
• Testing

Questions:

• Do we want to split off individual DE theme and detection logic into separate library files?