Sidebar buttons moved to headerbar

In the existing layout sidebar buttons (new game, pause, clear board and high scores) were taking big amount of screen space. When window had a vertical aspect ratio more space was wasted above and below the minefield. The loss of space was especially painfull on a smartphone screen.

This commit introduces changes in layout to minimize the loss of screen space:

• new game, pause and clear board buttons were moved to the headerbar
• high scores button was removed (the action remains available in the primary menu)
• clock and mines counter may be moved below the minefield if the window has vertical aspect ratio

Semi-related changes:

• primary menu definition was moved from vala to interface.ui
• minimum tile size was increased to 30