Add SPM support

[stjernegard]

This adds support for Swift Package Manager as a way to install the SDK in other projects.

• UIKit had to be imported in a lot of files that depend on things like UIApplication or UIColor.
• I've moved all resources into a /Resources/ folder because that made it easier to get all of them in Package.swift.
• The bundle that contains resources is different in SPM, so that's worth noting. See #if SWIFT_PACKAGE
• There's a few tests that aren't (always) passing in the SPM tests.
  • EventPublisherTests#testAddSubscriber is simply fragile because it has internal races. It seems to be equally fragile when running from the Xcode project.
  • Multiple tests in LocationServiceTests depend on the host app and test runner sharing a Keychain. That's not the case when running tests via SPM, but not an issue in real projects.
• CFBundleShortVersionString from Info.plist does not exist when running from SPM, so a new way to store the SDK version in code might be needed to keep the version tracking.

[ssathy2]

Thanks for the PR! I'll test it out and add feedback!

[ssathy2]

Looks good!