When the user clicks highlight(s) in the document and makes a "selection" in the
sidebar, we want the selected threads to always be visible. Previously this was
handled by clearing other types of filter when the selection was made, in
addition to setting the selection. This however meant that after the selection
was cleared, the state was different than prior to making the selection, with
any non-selection filters being disabled.

This commit changes the behavior so that the selection, if present, temporarily
overrides other filters rather than being combined with them. This allows the
selection to later be removed to revert back to the previous state.