osx-0.10.3: Add debug menu item to Dock App

It shows detected system load, with CPU count and memory metrics.

Except for that this release makes the Touch Bar app upgradable by
just launching the new Touch Bar installer. Before this change you
had to explicitly remove the previous one to be able to install
the upgraded one.

On the development side, this release comes with support for
Xcode 11.3 and an upgrade to Swift 5. The README info in the
osx/ folder has been moved into one README.md.