Entities that are "planted" on bodies of water are floating.
This does away with the "floating" mode.
To mark that entities should by default be floating when placed in
water the boolean property "floats" has been added.
The script is copied from the GoogleMock project and adapted
to instead generate stubs.
Two stub files are generated, one being named *_custom.h.
Any custom behaviour can be added to the latter, since it won't be
regenerated when the stub is recreated.