Suggesting new Event.object_type_to_color property

[mattdeitke]

For class-level (i.e., object type-level) segmentation, the documentation states:

Segmentation image by class of object (e.g. all mugs are the same color). Colors correspond to keys found in color_to_object_id. Only available when renderClassImage is enabled during Initialize call.

Though this is incorrect in (2) ways:

1. It doesn't make sense to use object_id's like CounterTop|+00.69|+00.95|-02.48, since the object types, not object instances, are the only things being color-coordinated.
2. When running

from ai2thor.controller import Controller
c = Controller(renderClassImage=True)
c.last_event.color_to_object_id

the color_to_object_id is always empty {}, which means color mappings are not provided for class level segmentation. However, when initializing with

from ai2thor.controller import Controller
c = Controller(renderClassImage=True, renderObjectImage=True)
c.last_event.color_to_object_id

the color_to_object_id are provided for BOTH the classes and object instances (though obviously runs substantially slower).

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Since there are only 125ish possible object types, which are always the same color across scenes (see #528.). My suggestion is to simply add a property to each Event called object_type_to_color that always return a complete cached color dict in the form

{'ScrubBrush': (222, 148, 80),
 'CoffeeMachine': (147, 71, 238),
 'RemoteControl': (187, 19, 208),
 'TissueBox': (98, 43, 249),
 'DishSponge': (166, 58, 136),
 'SideTable': (202, 45, 114),
# ........................... see #528 for the rest of non-interacted with types
 'Spatula': (30, 98, 242),
 'ShowerDoor': (36, 253, 61),
 'HousePlant': (73, 144, 213),
 'WineBottle': (53, 130, 252),
 'Statue': (243, 75, 41),
 'Bread': (18, 150, 252),
 'Painting': (40, 117, 236)}

A problem might arise across versions of AI2-THOR, where adding in new object types both changes the color scheme and requires the cached color dict to be updated. But, I suspect this isn't much of a problem if there are just a few tests that check for the number of objects in the scene, and the color dict works for a few images.