Fix clipping of GTK menu-bar labels for broad boards

Fix clipping of GTK menu-bar labels for broad boards

The number of characters kept in the menu-bar labels was dependent on
square size, but did not take into account the number of files of the
board. So when there were more than 8 files, the label texts were often
clipped while there was in fact plenty of room to display them fully.
Now the adjustment of square size based on number of board files is done
after the other window parameters have been picked based on the window
width calculated as 8*squareSize. Only the default line gap is then
adapted to the actual square size (consistent with what is done during
user sizing of the board later).