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/main.c

@@ -627,11 +627,13 @@ void os_exit(int r)
 {
   exiting = true;
 
-  if (!ui_client_channel_id) {
+  if (ui_client_channel_id) {
+    ui_client_stop();
+  } else {
     ui_flush();
+    ui_call_stop();
+    ml_close_all(true);           // remove all memfiles
   }
-  ui_call_stop();
-  ml_close_all(true);           // remove all memfiles
 
   if (!event_teardown() && r == 0) {
     r = 1;  // Exit with error if main_loop did not teardown gracefully.