refactor: Remove term modules and termcap options

- Removed term.c, term.h and term_defs.h
- Tests for T_* values were removed. screen.c was simplified as a
  consequence(the best strategy for drawing is implemented in the UI layer)
- Redraw functions now call ui.c functions directly. Updates are flushed with
  `ui_flush()`
- Removed all termcap options(they now return empty strings for compatibility)
- &term/&ttybuiltin options return a constant value(nvim)
- &t_Co is still available, but it mirrors t_colors directly
- Remove cursor tracking from screen.c and the `screen_start` function. Now the
  UI is expected to maintain cursor state across any call, and reset it when
  resized.
- Remove unused code

src/nvim/buffer.c

@@ -68,7 +68,7 @@
 #include "nvim/spell.h"
 #include "nvim/strings.h"
 #include "nvim/syntax.h"
-#include "nvim/term.h"
+#include "nvim/ui.h"
 #include "nvim/undo.h"
 #include "nvim/window.h"
 #include "nvim/os/os.h"
@@ -1420,7 +1420,7 @@ buflist_new (
     if (top_file_num < 0) {             /* wrap around (may cause duplicates) */
       EMSG(_("W14: Warning: List of file names overflow"));
       if (emsg_silent == 0) {
-        out_flush();
+        ui_flush();
         os_delay(3000L, true);          /* make sure it is noticed */
       }
       top_file_num = 1;
@@ -2159,7 +2159,7 @@ void buflist_list(exarg_T *eap)
         buf == curbuf ? (int64_t)curwin->w_cursor.lnum
                       : (int64_t)buflist_findlnum(buf));
     msg_outtrans(IObuff);
-    out_flush();            /* output one line at a time */
+    ui_flush();            /* output one line at a time */
     os_breakcheck();
   }
 }
@@ -2745,7 +2745,8 @@ static int ti_change(char_u *str, char_u **last)
  */
 void resettitle(void)
 {
-  mch_settitle(lasttitle, lasticon);
+  ui_set_title((char *)lasttitle);
+  ui_set_icon((char *)lasticon);
 }
 
 # if defined(EXITFREE)