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/ex_cmds.c

@@ -62,7 +62,7 @@
 #include "nvim/syntax.h"
 #include "nvim/tag.h"
 #include "nvim/tempfile.h"
-#include "nvim/term.h"
+#include "nvim/ui.h"
 #include "nvim/undo.h"
 #include "nvim/window.h"
 #include "nvim/os/os.h"
@@ -946,7 +946,7 @@ void do_bang(int addr_count, exarg_T *eap, int forceit, int do_in, int do_out)
     msg_putchar('!');
     msg_outtrans(newcmd);
     msg_clr_eos();
-    windgoto(msg_row, msg_col);
+    ui_cursor_goto(msg_row, msg_col);
 
     do_shell(newcmd, 0);
   } else {                            /* :range! */
@@ -1064,8 +1064,8 @@ do_filter (
   /* Create the shell command in allocated memory. */
   cmd_buf = make_filter_cmd(cmd, itmp, otmp);
 
-  windgoto((int)Rows - 1, 0);
-  cursor_on();
+  ui_cursor_goto((int)Rows - 1, 0);
+  ui_cursor_on();
 
   /*
    * When not redirecting the output the command can write anything to the
@@ -1245,23 +1245,18 @@ do_shell (
       }
     }
 
-  /* This windgoto is required for when the '\n' resulted in a "delete line
-   * 1" command to the terminal. */
-  if (!swapping_screen())
-    windgoto(msg_row, msg_col);
-  cursor_on();
+  // This ui_cursor_goto is required for when the '\n' resulted in a "delete line
+  // 1" command to the terminal.
+  ui_cursor_goto(msg_row, msg_col);
+  ui_cursor_on();
   (void)call_shell(cmd, flags, NULL);
   did_check_timestamps = FALSE;
   need_check_timestamps = TRUE;
 
-  /*
-   * put the message cursor at the end of the screen, avoids wait_return()
-   * to overwrite the text that the external command showed
-   */
-  if (!swapping_screen()) {
-    msg_row = Rows - 1;
-    msg_col = 0;
-  }
+  // put the message cursor at the end of the screen, avoids wait_return()
+  // to overwrite the text that the external command showed
+  msg_row = Rows - 1;
+  msg_col = 0;
 
   if (autocmd_busy) {
     if (msg_silent == 0)
@@ -1284,8 +1279,6 @@ do_shell (
        * want to wait for "hit return to continue".
        */
       save_nwr = no_wait_return;
-      if (swapping_screen())
-        no_wait_return = FALSE;
       wait_return(msg_silent == 0);
       no_wait_return = save_nwr;
     }
@@ -1945,21 +1938,6 @@ void viminfo_writestring(FILE *fd, char_u *p)
   putc('\n', fd);
 }
 
-/*
- * Implementation of ":fixdel", also used by get_stty().
- *  <BS>    resulting <Del>
- *   ^?		^H
- * not ^?	^?
- */
-void do_fixdel(exarg_T *eap)
-{
-  char_u  *p;
-
-  p = find_termcode((char_u *)"kb");
-  add_termcode((char_u *)"kD", p != NULL
-      && *p == DEL ? (char_u *)CTRL_H_STR : DEL_STR, FALSE);
-}
-
 void print_line_no_prefix(linenr_T lnum, int use_number, int list)
 {
   char_u numbuf[30];
@@ -1985,8 +1963,8 @@ void print_line(linenr_T lnum, int use_number, int list)
   print_line_no_prefix(lnum, use_number, list);
   if (save_silent) {
     msg_putchar('\n');
-    cursor_on();                /* msg_start() switches it off */
-    out_flush();
+    ui_cursor_on();                /* msg_start() switches it off */
+    ui_flush();
     silent_mode = save_silent;
   }
   info_message = FALSE;
@@ -3971,7 +3949,7 @@ void do_sub(exarg_T *eap)
               msg_no_more = FALSE;
               msg_scroll = i;
               showruler(TRUE);
-              windgoto(msg_row, msg_col);
+              ui_cursor_goto(msg_row, msg_col);
               RedrawingDisabled = temp;
 
 #ifdef USE_ON_FLY_SCROLL