[Dev] Sidebar Collections update

[John Anderson]

We had a meeting to discuss some changes to the Collections in the 
Sidebar today. Here's what we decided:

Well known collections in the sidebar, e.g. the Collection of 
Collections in the Sidebar, Trash, All, In, and Out should be directly 
accessible via python paths. Currently we are only doing this for Trash.

We'd like to move away from asking the Sidebar for information about 
Collections and instead just use the Collections themselves, or add new 
Collections that make it convenient to answer the questions about 
Collections. As an example of this, we are going to add a new 
KindCollection, which is a Collection of all the UserCollections. 
UserCollections are the kind of Collections that are added to the 
sidebar when you add a collection from the menu. UserCollection will be 
a subclass of InclusionsExclusionsCollection (suggestions for better 
names welcome). Currently, UserCollection's sole reason for existence is 
to be a Kind that is automatically added to the KindCollection of 
UserCollections, it will add no attributes or methods to 
InclusionsExclusionsCollection.

We will also add two new optional attributes to AbstractCollection: 
Color is the color of the Collection when it is seen in the sidebar and 
Calendar, and isShown, which allows us to hide In and Out in the Sidebar 
when the Calendar is visible. Originally we proposed adding these U/I 
attributes to another item that hung off the Collection, so we didn't 
pollute collectionness with U/I display attributes, however, that didn't 
allow FilteredCollection to filter out collections with isShown == 
False. We may change how we store U/I information in the future if this 
approach becomes awkward. Someday we may use multiple inheritance to 
construct a new Kind, SystemCollection, which would consist of Trash, 
All, In, Out, if treating these Collections as a group makes sense.

John