Cut in initial Xcode support for v6

[starkos]

What does this PR do?

Adds initial Xcode support to Premake6, bringing it up to level with the current Visual Studio exporter: it can generate Xcode projects for itself, which build and work. Only features required to meet that target have been implemented; much of the exported projects are still hardcoded.

How does this PR change Premake's behavior?

Adds a new xcode action. Fixes lots of bugs discovered along the way.

Anything else we should know?

• This includes fixes for a lot of issues I discovered along the way, not all of which were directly related to getting Xcode working. Let me know if you think it is better to just merge them as-is (faster) or break them out into separate commits (more "correct").
• I still need to fill in documentation for the new APIs which were added. I'm debating if I want to do that as part of this PR, or if it would be better to follow up with another, documentation-oriented commit where I just fill in as much as I can.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes (not really, see above)
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes (not really, see above)

You can now support Premake on our OpenCollective. Your contributions help us spend more time responding to requests like these!

[starkos]

FYI, I'm cutting a new branch to get started on filling in the documentation, so I'd prefer to handle that in a follow-up PR if there are no major objections.