refactor(ui): devirtualize the ui layer

- The defined interface for the UI is only the RPC protocol. The original
  UI interface as an array of function pointers fill no function.
- On the server, all the UI:s are all RPC channels.
  - ui.c is only used on the server.
  - The compositor is a preprocessing step for single-grid UI:s
- on the client, ui_client and tui talk directly to each other
  - we still do module separation, as ui_client.c could form the basis
    of a libnvim client module later.

Items for later PR:s
- vim.ui_attach is still an unhappy child, reconsider based on plugin experience.
- the flags in ui_events.in.h are still a mess. Can be simplified now.
- UX for remote attachment needs more work.
- startup for client can be simplified further (think of the millisecs we can save)

src/nvim/lua/executor.c

@@ -653,7 +653,7 @@ ok:
   }
 
   LuaRef ui_event_cb = nlua_ref_global(lstate, 3);
-  ui_comp_add_cb(ns_id, ui_event_cb, ext_widgets);
+  ui_add_cb(ns_id, ui_event_cb, ext_widgets);
   return 0;
 }
 
@@ -667,7 +667,7 @@ static int nlua_ui_detach(lua_State *lstate)
     return luaL_error(lstate, "invalid ns_id");
   }
 
-  ui_comp_remove_cb(ns_id);
+  ui_remove_cb(ns_id);
   return 0;
 }