XPChecklist - Checklist librar

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Added two new airplanes for XPlane:
-> Bombardier Challenger 300
-> Boeing 787 Dreamliner - provided by an user

Other changes
-> New design in homescreen
-> Rename menuitem 'Game' to 'Checklists' for better knowledge
-> added Back-Button in internal checklist creator (BETA) to go back to main menu
-> Updated Pictures in Google Play Store (external changes)