Don't attempt to activate the game if it has ended

The game is activated in flowers_game_tile_state_changed, and only if
the game is not already active. When the game ends it is however first
disabled, and then the remaining tiles are pressed. This will cause the
game to be set to active again. Also check that the game is in the none
state, that it has not been set to won or lost.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>