Split the core API in to its own shared library

[Chris Coulson]

I want to experiment with a fix for bug 1607411, but to do this I need to make a fairly large architectural change. Essentially, I want to:

- With the exception of qt/core/api, rename qt/core to qt/lib
- Rename qt/lib/glue to qt/lib/public (it's only "glue" because "public" is too similar to "api").
- Route all dependencies from qt/core to qt/lib via qt/lib/public (similar to how qt/quick works).
- Break all dependencies from qt/lib to qt/core
- Build qt/lib as a new shared library, installed in to libexecdir (liboxideqt.so, or something). oxide-renderer will link against this new library.
- qt/core still creates libOxideQtCore.so.0, but build it with CMake and (for now) have it link against the new library.