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/ui_client.h

@@ -34,8 +34,6 @@ EXTERN TriState ui_client_bg_respose INIT(= kNone);
 /// by convention, this uses fd=3 (next free number after stdio)
 EXTERN bool ui_client_forward_stdin INIT(= false);
 
-EXTERN char *ui_client_termname INIT(= NULL);
-
 #define UI_CLIENT_STDIN_FD 3
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ui_client.h.generated.h"