Refactor Layer/ColorLayer into siblings

New class hierarchy should look like:

           Layer (abstract)
           /   \
          /     \
        Buffer  Color
        Layer   Layer

Note: LayerBuffer is what previously was referred to by class Layer

Test: Transaction_test.cpp/manual test

Change-Id: I372a27d533e08371da37be811e4372404f7787bd
10 files changed