This no longer is just private API of the file code as we're using this
all over balooctl and now QML plugin.
The relevant namespace is exported and exposed to QML. Intended to help
tidy up the patch D25447 that horribly shadows the enum values manually.
I wanted to avoid linking baloofilecommon as the qml plugin doesn't
really use anything from that, so I moved the enum and relevant method
into indexerstate.