vim-patch:9.1.0116: win_split_ins may not check available room

Problem:  win_split_ins has no check for E36 when moving an existing
          window
Solution: check for room and fix the issues in f_win_splitmove()
          (Sean Dewar)

0fd44a5ad8

Omit WSP_FORCE_ROOM, as it's not needed for Nvim's autocmd window, which is
floating. Shouldn't be difficult to port later if it's used for anything else.

Make win_splitmove continue working for turning floating windows into splits.
Move the logic for "unfloating" a float to win_split_ins; unlike splits, no
changes to the window layout are needed before calling it, as floats take no
room in the window layout and cannot affect the e_noroom check.

Add missing tp_curwin-fixing logic for turning external windows into splits, and
add a test.
NOTE: there are other issues with the way "tabpage independence" is implemented
for external windows; namely, some things assume that tp_curwin is indeed a
window within that tabpage, and as such, functions like tabpage_winnr and
nvim_tabpage_get_win currently don't always work for external windows (with the
latter aborting!)

Use last_status over frame_add_statusline, as Nvim's last_status already does
this for all windows in the current tabpage. Adjust restore_full_snapshot_rec to
handle this.
This "restore everything" approach is changed in a future commit anyway, so only
ensure it's robust enough to just pass tests.

Keep check_split_disallowed's current doc comment, as it's actually a bit more
accurate here. (I should probably PR Vim to use this one)

Allow f_win_splitmove to move a floating "wp" into a split; Nvim supports this.
Continue to disallow it from moving the autocommand window into a split (funnily
enough, the check wasn't reachable before, as moving a float was disallowed),
but now return -1 in that case (win_splitmove also returns FAIL for this, but
handling it in f_win_splitmove avoids us needing to switch windows first).

Cherry-pick Test_window_split_no_room fix from v9.1.0121.

Update nvim_win_set_config to handle win_split_ins failure in later commits.

src/nvim/api/win_config.c

@@ -526,18 +526,6 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
       }
     } else {
       win_remove(win, win_tp == curtab ? NULL : win_tp);
-      ui_comp_remove_grid(&win->w_grid_alloc);
-      if (win->w_config.external) {
-        for (tabpage_T *tp = first_tabpage; tp != NULL; tp = tp->tp_next) {
-          if (tp == curtab) {
-            continue;
-          }
-          if (tp->tp_curwin == win) {
-            tp->tp_curwin = tp->tp_firstwin;
-          }
-        }
-      }
-      win->w_pos_changed = true;
     }
 
     int flags = win_split_flags(fconfig.split, parent == NULL) | WSP_NOENTER;