Updated buttons to match the mockups

* Updated the buttons to match the mockups
* Set a sane minimum size for the window
* Reduce spacing between the new game screen buttons to 18px

https://bugzilla.gnome.org/show_bug.cgi?id=727960
Set a reasonable size request for the main window

Otherwise, the window can be resized to excessively small sizes. Besides
being unuseful in its own right, this causes the window to expand when
pausing the game, since pausing the game adds more buttons.

https://bugzilla.gnome.org/show_bug.cgi?id=727960