macOS updates

[MaddTheSane]

• Remove old Xcode/ProjectBuilder build files.
• Migrate from ProjectBuilder project to Xcode project.
• Use Xcode assets for the icon
• Convert nibs to xibs: They're more diff-friendly.
• Remove big-endian defines: x86_64 and ARM64, the current targets, are little-endian.
• Convert a lot of the Cocoa code to ARC.

This is based off of my work here: https://github.com/MaddTheSane/arcem
There's no guarantee that this will work, but it does build (when using Xcode).

[ccawley2011]

Looks good so far, although it doesn't start for me at the moment.

I've left some initial review comments - some of it is likely needed to get it working, while others are just suggestions to make future maintenance easier.

[ccawley2011]

Given that the keyboard emulation code is now using a circular buffer internally, it should hopefully be possible to get rid of the second circular buffer in the frontend code.

[ccawley2011]

With a few extra modifications, I can now get the main window to open and access the menu options. The graphics for the main display don't work however, but you can still lock the mouse pointer and see the cursor graphic moving.

[ccawley2011]

I've pushed some changes to the branch, which allow video output to work correctly.