Create and install CMake Config file for marble & astro libraries

Summary:

• use "Marble" and "Astro" as cmake target names, so 3rd-party lib users would do: find_package(Marble REQUIRED) find_package(Astro REQUIRED) target_link_libraries(foo Marble Astro) # include dirs imported by target
• only install headers & cmake config files on unbundled platforms for now

Reviewers: rahn, sanjibanb, shentey, nienhueser, Marble

Reviewed By: nienhueser, Marble

Differential Revision: https://phabricator.kde.org/D2364