Specify tab indentation in xcode projects (lib and templates)

This PR specifies tab indentation style for the OF xcode project, as well as the project template (both osx and iOS). This is [as discussed over on this commit](https://github.com/openframeworks/openFrameworks/commit/cd9fb7d95264af88038839220d58bad11aae5ae0#commitcomment-8397854)

Setting tab indentation style in the project template might be a little contentious, since it should override the user's xcode preferences when working on OF apps. If we decide against that I can submit a PR with just the OF project itself modified.

ping @openframeworks/ios @openframeworks/macos