rework EntitySelection to require explicit list of selected properties

[kukeiko]

What

Currently, the EntitySelection allows as a value an object that does not explicitly list all properties that are to be selected. When it comes to performing operations (subtract/merge/intersect), it'll query the EntitySchema for the default selection when necessary.

The reason I did it this way (that change is actually quite recently and wasn't like this for a long time before that) was to make construction of EntitySelections easier for the user.

Why

Several reasons:

• impossible to represent a partial EntitySelection (i.e. if "Id" & "Name" is required in Entity, no way to just select "Id")
• it makes the code of operations (subtract/merge/intersect) quite a bit more complex
• harder to grasp mentally what the difference between two EntitySelections is
• ease of EntitySelection creation by the User can be done outside the class, making EntitySelection itself simpler
• it requires that the EntitySchema exists and is fully described (at least the parts the EntitySelection targets) when operations are executed

[kukeiko]

merging, subtraction & intersection logic now relies on selected being explicitly listed, so we can close this