fix(api): handle win_split_ins failure properly

Problem: nvim_win_set_config does not handle failure in win_split_ins properly
yet, which can cause all sorts of issues. Also nvim_open_win and
nvim_win_set_config do not set the error message to the one from win_split_ins.

Solution: handle failure by undoing winframe_remove, like in win_splitmove.
Make sure autocommands from switching to the altwin fire within a valid window,
and ensure they don't screw things up. Set the error message to that of
win_split_ins, if any.

Also change a few other small things, including:

- adjust win_append to take a tabpage_T * argument, which is more consistent
  with win_remove (and also allows us to undo a call to win_remove).

- allow winframe_restore to restore window positions. Useful if `wp` was in a
  different tabpage, as a call to win_comp_pos (which only works for the current
  tabpage) after winframe_restore should no longer be needed.

  Though enter_tabpage calls win_comp_pos anyway, this has the advantage of
  ensuring w_winrow/col remains accurate even before entering the tabpage
  (useful for stuff like win_screenpos, if used on a window in another tabpage).

  (This change should probably also be PR'd to Vim later, even though it doesn't
  use winframe_restore for a `wp` in a different tabpage yet).

src/nvim/winfloat.c

@@ -61,7 +61,7 @@ win_T *win_new_float(win_T *wp, bool last, WinConfig fconfig, Error *err)
     XFREE_CLEAR(wp->w_frame);
     win_comp_pos();  // recompute window positions
     win_remove(wp, NULL);
-    win_append(lastwin_nofloating(), wp);
+    win_append(lastwin_nofloating(), wp, NULL);
   }
   wp->w_floating = true;
   wp->w_status_height = 0;