Improve UI to support dark mode

[zenangst]

Description

• The list items now have less height to fit more applications on screen, this results in less scrolling when using the application
• The search field is now styled after the upcoming macOS Catalina release, this is mostly done by reducing the amount of corner radius that the view gets
• The application now supports both light and dark appearance
• The side menu (the application list) now uses a NSVisualEffectView
• The feedback button now uses a regular NSButton

Screenshots

Dark mode

Light mode