budgie-plugin: Add+install Gir, Typelib, vapi to host for plugin devving

We now ship our header so that developers can create a plugin for Budgie,
currently limited to the panel, in any GIR capable language, including C.
A vapi file is also shipped for Vala developers, and the typelib/Gir
combination will enable Python, JS, etc, developers to create plugins
too.