Bug fixes and new features

Fixed wrong margins and transparency in test string rendering.
Added ability to re-open the default font using File->New.
Added ability to copy the current character.
Added ability to copy/paste the current character image (pasting images
from other applications does not work yet).
When drawing multiple pixels fast there will be no gaps between them
anymore.
Window title now contains the font filename as soon as a font has been
opened or saved.