ENH: Re-use graphical interface window from trivial game #80
Description
At the moment, the sequence (1) launch Gambit and then (2) open a game file results in two Gambit windows, the first containing a trivial game and the second the opened game.
Consider re-using a window containing a pristine, unedited "trivial" game -- if one is present -- to load a new game file, to minimise window clutter.