Workspace/Project Updates

[stevenzeck]

This PR does several things:

1. Moves the tests from TestApp over to Playground.
2. Adds support for LCP in Playground. Run make playground lcp=<url> in the root.
3. Enables LCP tests both locally and in CI. When you run make playground, LCP tests are not included. When you run make playground lcp=<url>, they are included. Note that they are not included in Readium-Package. They need a host app for Keychain, which is why they're tied to the Playground.
4. Adds a workspace file to have Readium, Playground, and TestApp open at once. You still need to run the make commands for Playground and TestApp, this does not replace that. It's a single file that can easily be deleted once the TestApp is removed.
5. Adds a cache for CI builds.