Misc improvements

[btoews]

This PR makes a few changes. Let me know if you'd like me to split this up into multiple, smallers PRs.

• Adds a .gitignore file to avoid checking in unnecessary changes to the Xcode project.
• Adds targets to the Xcode project for the examples (keyboard/mouse).
• Fixes up the keyboard example to cleanup the virtual keyboard when the program exits.
• Extracts constants used in kernel and userland into foohid_user_kernel_shared.h
• Adds externally callable methods for opening/closing the user-client more cleanly.
• Adds some more cleanup code to the user client so it gets properly removed from registry when finished.

I also cleaned up a bunch of garbage whitespace throughout the project. This makes the diff kind of hard to read on GitHub. Appending ?w=1 to the URL hides the whitespace changes from the diff, making it easier to read.

I'll be opening a separate PR to implement the setReport stuff.

[btoews]

Here's a branch adding setReport support. It's based on this branch, so I can't cleanly open a PR until this one merges.

[aldur]

Thanks! I reviewed the changed files, but I didn't test the new kext (I don't have a macOS install available ATM). @unbit, what do you think?

[unbit]

The patch seems good but i need to test it :) I will check it during the week-end

[btoews]

@unbit Cool, thanks. Would you like me to include the setReport changes in this PR or do that in a separate PR once this one merges?

[btoews]

This PR ended up mostly being aesthetic changes that aren't really important. I opened another PR (https://github.com/unbit/foohid/pull/9) to implement the setReport call, which is what I'm really interested in :smile:. I'm closing this PR for now.