Added initial support for dragging ResourceItems. This is achieved by
using drag'n'drop support offered by Qt's model/view technology.
Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
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>