main: Fix color schemes for abstract_ui

- Set 't_Co' to 256 at startup. The value can be changed by the user for
  compatibility with terminals that are less capable.
- `has('gui_running')` will return 1 if at least one rgb UI is attached.

Even though these changes are hacky, they are necessary to make the transition
to the new UI architecture smoother.

src/nvim/screen.c

@@ -6150,8 +6150,7 @@ void screen_fill(int start_row, int end_row, int start_col, int end_col, int c1,
     return;
 
   /* it's a "normal" terminal when not in a GUI or cterm */
-  norm_term = (
-    t_colors <= 1);
+  norm_term = (!abstract_ui && t_colors <= 1);
   for (row = start_row; row < end_row; ++row) {
     if (has_mbyte
         ) {
@@ -6675,8 +6674,8 @@ static void linecopy(int to, int from, win_T *wp)
  */
 int can_clear(char_u *p)
 {
-  return *p != NUL && (t_colors <= 1
-                       || cterm_normal_bg_color == 0 || *T_UT != NUL);
+  return abstract_ui || (*p != NUL && (t_colors <= 1
+        || cterm_normal_bg_color == 0 || *T_UT != NUL));
 }
 
 /*