Add tests...

This would really speed up development due to a reduction in manually testing everything before pushing a new feature :)

Tests that I can think of. Post a comment if I missed one.

• All swaync-client args
• All subscription actions output a correct swaync-client subscription value
• Blank windows being shown and not opened on the focused monitor
• Notification actions
• DND with different notification urgencies
• Scripts
• Keyboard shortcuts (if this even is possible...)
• Styling if this even is possible (maybe screenshots with manual approval???☹️)
• Verify notifications moving from NotificationWindow to the Control Center when cc is opened
• Notification icon priority (provided -> app icon -> desktop entry icon -> default icon)
• Notification icon visibility depending on config setting
• Body markdown
• Body image
• CC margin and positioning
• Notification timeouts
• Notification visibility
• Config and CSS expand paths (start with "~/", "/" or "./")
• Config and CSS loading
• Config values