Added Searcher.isIndex(dir) to check if a directory is a BlackLab index.

Used by BlackLab Server to scan collection directories for valid
indices.
Added method to support deprecated BlackLab Server setting.

IndexStructure._setPidField() should not be called by other
applications. The setting in question already triggers an error in the
application log and will be removed from BlackLab Server in a future
version.
Added contentViewable metadata setting.

This setting indicates whether or not all end users may retrieve the
full contents of documents. Note that BlackLab itself does not use this
setting; it exists merely to provide a simple way for applications to
implement "all or nothing" access controls. BlackLab Server respects
this setting and will refuse to return the full contents of a document
if the setting is missing or set to false for an index.
Made sure ContextQL parser accepts all valid properties.