Fixed the Sudoku crash on starting Hard/Very Hard games

According to the docs, when the MapIterator is out of track,
neither get_key, get_value, set_value nor unset are defined
and all will fail.
After the next call to next(), they will be defined again.

When the MapIterator is initialized, it is out of track.
Because of it, the assertion to check that failed,
and it caused the crash.
This patch advances the iter by calling next()

https://bugzilla.gnome.org/show_bug.cgi?id=633464
Fixed the GLib Critical error related to GLib.Timer

.continue() resumes a timer previously stopped with .stop()
But since .stop() is not called anywhere in the code,
the assertion to check that failed and raised the error.

Since we haven't implemented Pause and Resume functions,
and start_game() is meant only for starting a game,
I have removed the timer.continue() call.
When we implement those functions,
timer.stop() and timer.continue() will be used in them

https://bugzilla.gnome.org/show_bug.cgi?id=633464
Updated TODO : Removed the two bugs which have been patched

https://bugzilla.gnome.org/show_bug.cgi?id=633464