While loading a blend file this patch closes any open menus and changes
the mouse cursor to "wait".

Loading a blend file can sometimes make the system look unresponsive. If there is a menu up at the time (like if you select from File/Recent) it will remain frozen onscreen until the file is loaded:

Although the mouse cursor initially changes to a "wait" cursor, this is reset back to a normal pointer very early on in the process. These things both make the program look like it is hung. And not in a good way.

This patch just removes any menus that might be showing, and also ensures that the "waiting" mouse cursor continues to show until (almost) the end.

This is a subset of D10484. It makes sense that we might debate whether or not we want a "loading" dialog and what it might look like, but that can be separate from this.