Move the minefield to an aspectframe. https://bugzilla.gnome.org/show_bug.cgi?id=728483

Moved the minefield to an aspectframe correctly maintaining the
aspect and centering of the minefield, and removed the hard-coded
calculations from the minefield class.
Use standard GTK+ components for the minefield
Use the SVG files as background-image
Added pause overlay
Fix paths to image files in css
Merged branch 'wip/cssstyling' with CSS stylable minefield.
https://bugzilla.gnome.org/show_bug.cgi?id=728483