Workspaces and selective aggregated mode

Right now, we have aggregated mode which allows to open all roots and select resources from all of them at the same time. In future, we will have saved queries section (#80). For a saved query, it would be nice to support several chosen folders to run the query on. This means, we need to support usage of several prefixes as joint filter at once.

Start by tracing the existing aggregated mode and how it opens roots and selects resources, then review the planned saved queries work in issue #80. Define how several chosen folders become prefixes in one joint filter, and verify that a saved query runs only against those folders while aggregated mode continues to work.

Written by the indexing model from the issue text.