Remove the preferences dialog

We only have two user-visible settings. They can more comfortably live
in the app menu.
Fixed various sidebar button issues https://bugzilla.gnome.org/show_bug.cgi?id=727960

* Reworded Change Difficulty button to Difficulty
* Added button padding to match the mockups
* Do not show the Difficulty button between game start and timer start
Updated Greek translation
Updated Greek translation
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
Added new co-maintainer
Merged branch 'wip/cssstyling' with CSS stylable minefield.
https://bugzilla.gnome.org/show_bug.cgi?id=728483
Updated Czech translation
Updated Czech translation
Added new classes for game ended cases
Credits cleanup, removed face artists, as we are not using faces.
https://bugzilla.gnome.org/show_bug.cgi?id=727182
Mark the full field as exploded instead of separate tiles
Prefer dark gtk theme

https://bugzilla.gnome.org/show_bug.cgi?id=728261
Moved start screens to Glade UI file
Moved most of the game screen to the UI file
Further improvements to the UI file