windows: float config changes

- Allow floating windows of width 1. #9846
- For a new floating window the size must be specified. Later on we
  might try to calculate a reasonable size by buffer contents
- Remember the configured size of a window, just like its position.
- Make get_config and set_config more consistent. Handle relative='' properly in set_config.
  get_config doesn't return keys that don't make sense for a non-floating window.
- Don't use width=0 for non-changed width, just omit the key.

src/nvim/screen.c

@@ -363,7 +363,7 @@ void update_screen(int type)
     need_wait_return = FALSE;
   }
 
-  if (type >= NOT_VALID) {
+  if (type >= CLEAR || !default_grid.valid) {
     ui_comp_set_screen_valid(false);
   }
   win_ui_flush_positions();