Added some entity movement code in genericEntity and Spider.

Early, basic changes so far.
Checking that git works again inside Android Studio
Implementing the gravity function for falling objects.
Added a GenericItem class to assist with objects which can fall down under gravity.
Bomb.java now extends GenericItem instead of GenericEntity.  It contains a partial onTick() which deals with explosions and falling.  It contains an overriding gravity() to prime the bomb for explosion.
Boulder.java now extends GenericItem instead of GenericEntity.
Diamond.java now extends GenericItem instead of GenericEntity.
Dirt.java now has booleans for space around the dirt for sprite purposes, and a refreshDirt() to refresh those booleans.
Gem.java now extends GenericItem instead of GenericEntity.
GenericEntity no longer contains gravity().
GenericItem was created and contains gravity().

As usual I have no idea what I'm doing, so there are lots of TODO statements which involve checking things for validity.