Add group bitmask filtering for PhysicsCollisionObject

PhysicsCharacter, PhysicsGhostObject and PhysicsRigidBody now supports
collision filtering using bitmasks.

See issue #1029 on github.
Fixed compilation error on Android

Without the explicit cast call to std::string::insert(size_t, size_t, char)
becomes ambiguous.
fixed early exit when iterating Theme overlays
Changed shorts to ints for collision group bitfields.
Android build: Clean assets folder before building package

Otherwise deleted files from the project remain in the built .apk archive.
Merge pull request #1053 from rgngl/physicsFilters

Add group bitmask filtering for PhysicsCollisionObject
Merge pull request #1055 from rgngl/androidFix

Fixed compilation error on Android
Merge pull request #1060 from rgngl/androidBuildClean

Android build: Clean assets folder before building package
Merge pull request #1061 from blackberry/master

Merge master to next
Merge pull request #1059 from reven86/next-clean

fixed early exit when iterating Theme overlays
Merge pull request #1062 from blackberry/master

Merge master to next