BUG: Avoid unneeded copy of resource bytes

Modify qSlicerCoreApplication::[un]registerResource to use the
QByteArray's constData() rather than mutable data() in order to avoid an
unnecessary detach of the internally shared data. (Our usage does not
require a mutable pointer at any time.)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23299 3bd1e089-480b-0410-8dfb-8563597acbee