Simplify selection parsing methods signature.

Simplify selection parsing methods signature.

Selection parsing methods in SelectionCollection now return the list of
parsed selections instead of using an output variable.  This makes it
more obvious from the signature alone how the methods should be used.
Also, the ability to append to an existing vector was not used in any
code currently (except in tests), and the code to do that with the new
signature is still simple.

Change-Id: I344e4c15367e956b4294d18f56f4da34ccf42e27