The internal function that filters annotations returned a list of matching IDs.
Unsaved annotations do not have IDs, so were excluded from the matches. Change
the function to return a list of matching annotations instead, lifting this
restriction.

No callers of the function had to change because currently they only test the
length of the returned array.

A caveat with this change is that it only matches the data stored in the
annotation object, not data stored in separate "draft" objects which are
combined when rendering unsaved annotations. The result is that a filter will
only match the fields of the annotation that are populated when the new
annotation initially appears in the sidebar. This for example includes the user
and page number, but not text added in the text field later.