Save font settings based on initial square size

Save font settings based on initial square size

Font settings were saved by the square size as it was at the time of
saving, which could have been altered compared to the reading of the
fonts from the settings file by interactive sizing of the window, or
by switching to a variant with other board dimensions. Now the initial
square size is remembered and used for this.
BEWARE: we might want to undo this when window sizing would affect font
selection, rather than just sizing the board.