Depend on GTK+ 3.10

For GtkHeaderBar
Use a GtkHeaderBar
Remove fullscreen mode
Replace the toolbar with a gear menu
Remove superpaused state

This wasn't a very great idea. It also lets you cheat by pausing the
timer without running down the clock. People can simply take the time to
press pause if they want to pause the game.
Simplify pause code for action/widget sensitivity
trivial: use C-style casts to retrieve GMenus

* No point in using the as keyword if we're not going to test the result
  against null
* Let's use GMenu instead of GMenuModel
Swap Pause/Resume menu label as appropriate

Unfortunately, this requires dynamically generating the menu. I'm not
very impressed with the GMenu API compared to the rest of GLib....
Restore keyboard accelerators
Remove old workaround for bgo#647122
Rename save_game() to present_save_dialog()
Rename save_cb() -> save_dialog_cb()
Remove save_file_cb()
Reorder and reposition the save game functions
Add an accelerator for Pause/Resume
Use the current filename as the headerbar subtitle
Never autosave over a user's files

We should only ever be autosaving to the single autosave file.
Remove redundant saved_filename variable
Remove an unneeded function call
Add an accelerator for Save As

Shift+Ctrl+S