Playtime: Add Linux support - Stage 1

[helgoboss]

The goal is to run Playtime Engine and the Helgobox App on Linux. This will happen in 2 stages. Stage 1 comes with the limitation that the Helgobox App will only work standalone as a separate main process and window, not yet integrated into REAPER. It communicates with the plug-in over a local network TCP/IP connection

I did initial tests and identified the following todos.

Primary todos:

• GitHub Actions: Enable feature playtime and licensing when building the plug-in on Linux (x64 and arm64)
• Extend helgobox-devtool to support publishing Helgobox App from Linux (x64 and arm64)
• Enable code blocks which are currently disabled on Linux (x64 and arm64)
• Add ReaPack files (x64 and arm64)
• Look into shipping as combined shared library and executable
• Make Helgobox App connection configurable and encode desired instance as part of the location
• Improve Helgobox App command line usage
• Implement thread priority stuff correctly
• Fix emojis in App:
  • Purchase button
  • Settings screen arrow
  • Fader
• Fix issues with gRPC server seemingly sending data in bursts (it's not a client issue!) => This turned out to be a temporary issue on the VM
• Save/load configuration (to/from user home)
• Adjust max thread priority value (await Justin's response) (moved to stage 2)
• Actually publish Linux version of Helgobox App (x64 and arm64)

Secondary todos:

• Make the Helgobox extension work on Linux (loading plug-in library in advance)
• Implement mouse cursor locking for Linux
• Improve mouse cursor locking on Linux (right now, it grabs the cursor a bit slowly after starting too drag) (moved to stage 2)

Nice-to-have todos:

• Improve stage 1 message when clicking Playtime button on Linux