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/quickfix.c

@@ -47,7 +47,7 @@
 #include "nvim/screen.h"
 #include "nvim/search.h"
 #include "nvim/strings.h"
-#include "nvim/term.h"
+#include "nvim/ui.h"
 #include "nvim/tempfile.h"
 #include "nvim/window.h"
 #include "nvim/os/os.h"
@@ -1786,7 +1786,7 @@ void qf_list(exarg_T *eap)
           ? skipwhite(qfp->qf_text) : qfp->qf_text,
           IObuff, IOSIZE);
       msg_prt_line(IObuff, FALSE);
-      out_flush();                      /* show one line at a time */
+      ui_flush();                      /* show one line at a time */
     }
 
     qfp = qfp->qf_next;
@@ -2845,7 +2845,7 @@ void ex_vimgrep(exarg_T *eap)
       msg_didout = FALSE;           /* overwrite this message */
       msg_nowait = TRUE;            /* don't wait for this message */
       msg_col = 0;
-      out_flush();
+      ui_flush();
     }
 
     buf = buflist_findname_exp(fnames[fi]);