Add modelines to all Vala files
Require GTK+ 3.13.2

For automatic RTL icon support
Use a GtkStack for the play/pause buttons
trivial: set grid margin in UI file
Update button layout for new design
Double min tile size

Larger tiles are more similar to the new design, and easier to play with
on a touchscreen.
Add a warning dialog when using solve game

Since this ends the user's game, he should have a chance to change his
mind.
Remove unused signal PuzzleView.tile_moved()
Start the clock immediately on new game

In some games, it makes sense to start the clock after the first move
has been completed. Tetravex is different: the first move requires a lot
of thought, and it ought to be counted against the player's time.