Increase spacing between new game buttons

As per the new designs

https://bugzilla.gnome.org/show_bug.cgi?id=727959
Use "delay-apply" mode for GSettings

By delaying the settings, we will not lose any values within the current
session.

https://bugzilla.gnome.org/show_bug.cgi?id=727191
Custom game screen improvements. https://bugzilla.gnome.org/show_bug.cgi?id=710235

Improved custom game screen by:
* rewording fields based on mockup
* rearranging buttons
* centering the whole custom game screen
* adding suggested-action class to the play button
Use headerbar in the preferences dialog

https://bugzilla.gnome.org/show_bug.cgi?id=727050
Require GTK+ 3.12, Vala 0.24

For GtkDialog use_header_bar

https://bugzilla.gnome.org/show_bug.cgi?id=727050
Updated Hebrew translation
Never pause the game before it has begun

Clicking outside the window incorrectly results in the New Game button
being replaced with the Play button, which both pauses and resumes the
game. Oops.

https://bugzilla.gnome.org/show_bug.cgi?id=728305
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
Replace the hint button with a flag icon

As per the new design. (The flag icon is temporary.)
trivial: remove unneeded variables
Custom games: remove num mines spinner

We currently have a spinner to set the number of mines and a different
spinner to set the percentage of mines. These are redundant, and I like
the percentage spinner more, so remove the original one.

https://bugzilla.gnome.org/show_bug.cgi?id=710235