The new generic ItemRelation template class replaces ResourceTreeItem's,
resulting in cleaner and better organized code.
In the future other kind of items can be linked hierarchically using
the ItemRelation template.
Naming classes like ResourcesItem or ResourcesDatabase is bad style,
therefore rename all classes and files to be Resource*.
Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>