Thread safey in the Query Service

The Nepomuk::Query::Folder::initialListingDone() function can be called via dbus
at any point via the FolderConnection, at that point the listingFinished() function
might be in the process of changing m_initialListingDone's value => CRASH

Ditto with Nepomuk::Query::Folder::entries(). It can be called via
FolderConnection's list method.

BUG: 269650
REVIEW: 102129