Change the way objective is displayed in Main game

The small amount of time used by Paul at each start/restart of the Main game becomes a bit annoying when we try many times in a row. So we may use this way only the first time a new objective is given, and all other times using a less intrusive way.

I think about :

• display the objective on the top of the screen (in the middle)
• remove it just after the plane fly away (using a tween motion to move it above the screen)