🖥️ Improved Window System & Input

[jameskr97]

GLFW to replace LowLevelWindow

This PR was originally attempted in #996 though, when I was originally working on it, that branch took several different directions, and I thought too much was happening at once. Those changes will still be coming, but one at a time rather than all at once. This PR is created specifically for the Windowing system replacement, with the organization to only StepMania.{cpp,hpp}.

Changes Include

• Organizing StepMania.{cpp,h}
• glew replaced with glad
• macOS and Linux InputHandler files are being deleted in place of GLFW.
• Removed src/arch/LowLevelWindow
• Support for retina displays by using frame-buffer size instead of window size for GL operations. (FrameBuffer typically matches window dimensions on non-retina displays). The top bar is also no longer blurry.

PR to resolve the following issues:

• Closes disable vblank_mode on linux #906. Linux only uses OpenGL, and it's disabled via the glfwSwapInterval(0)
• Closes Function keys in macOS trigger events in Etterna without the fn key being held down #784. GLFW properly handles the fn key, and the fn key is no longer usable in-game.
• Closes Right-Clicking Does Not Work With MacOS #829. Issue no longer present in this PR.
• Closes X11 uptime (for 49 days) causes overflow, breaking input #655. X11 timestamp no longer relied upon for input time
• Closes Setting mode to fullscreen in settings makes Etterna black screen on macOS. #940. macOS fullscreen is not black anymore in this PR. The "Fullscreen" option doesn't show up. When the fullscreen button is clicked, the game doesn't go black.
• Closes Etterna on macOS is not responding to any keyboard inputs or mouse clicks #1047. Currently, we have an API that requires special permissions. GLFW doesn't use that.

[jameskr97]

Before this pr:

After this pr:

The change here is unacceptable, and I will start over.