Linux: Build in one-dir mode and use AppImage tools

[Avasam]

Instead of using PyInstaller's onefile mode (self-extracting-executable) on Linux, we should build in one-dir.

This has the following advantages:

• Use a standard "portable single executable" on Linux
• Ship with an icon (Generate a Desktop icon on Linux #274) and allow users to use AppIamge-related tooling (like auto-registration as apps)
• Better UPX compression