Fix expose of to-square with grid off

The code relied on drawHighlight to request an expose evet for the entire
square, but when lineGap = 0, drawHighlight is a no-op.
Fix warning in WinBoard
Let WinBoard start in its installation folder

The current directory in WinBoard was not automatically the same as the
installDir returned by SearchPath. Especially not if used without shortcut
by file association.
Assign shortcut char to WB menu item

The new Save games as Book item did not have a shortcut yet.
Add some new strings to WB translation template
Update Dutch WB translation
Fix GTK error auto-raising board

The variable mainwindow is not set in the gtk2 version. Unfortunately
it is still used to determine default board size...