Refactor GravityAction

Remove testPositiveYVelocity()
Add testVaryingGravity()
Add helper-methods
Merge pull request #1098 from Catrobat/CAT-1117-GravityAction

Refactor GravityActionTest
CAT-1116 #done physics glide to behaviour
Merge pull request #1095 from Catrobat/physics_CAT-1116_changeGlideToPhysicsBehaviour

CAT-1116 #done physics glide to behaviour
CAT-1083: active physics stage area incl. rudimentary tests

checking whether physicsObject in active area only once for X/Y using circumference of its AABB now
checks now performed in more critical methods
added file for test with huge sprite (8192x8192) exceeding most devices' active physics stage area
fixed circumference test to allow for small anomalies (tested recangle has size of 124x124 instead of 125x125)
using PhysicsWorld for definition of activeArea