Add minimal app switcher protocol

This adds a simple private protocoll to list and raise toplevels by app_id and title. We wanted a minimal set of objects, requests and events. Since this is only meant for the app overview page in the shell we can take many shortcuts and leave out e.g. XWayland for the moment.

We will switch to wlr_foreign_toplevel_unstable_v1 once that is merged.

The first commit just renames stuff the second adds the functionality.

To test you need phosh's app-switcher branch. The apps will be shown in the overview (when hitting the home button).