Workspaces and selective aggregated mode

[kirillt]

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.

[kirillt]

We should introduce something like this:

interface Workspace {
    fun folders(): List<Path>
}

data class SingleFolderWorkspace(val root: Path, val subfolder: Option<String>) {
    override fun folders(): List<Path> { ... }
}

data class MultipleFoldersWorkspace(val roots: List<Path>) {
    override fun folders(): List<Path> { ... }
}