GUI: Fix for bug 6719 (last line of help missing)

Since 1.4 the font changed from a 14 to 15 pixel height wich causes
one line of help text no longer to be shown. By changing the padding
of the buttons the last lines fits again.
Merge pull request #503 from BenCastricum/fixforbug6719

GUI: Fix for bug 6719 (last line of help missing)