Advanced value editors don't work in NewEntryWizard

Some advanced value editors, that use the connection object, don't work in the NewEntryWizard. The cause is that we use a DummyConnection that only provides the schema information of the real connection, but it doesn't support any operation on the directory. The "DN Editor" however needs the real connection to be able to browse the tree. The reason why this DummyConnection is used is that the modifications in the NewEntryWizard shouldn't be send to the directory before the "Finish" button is pressed, because the entry doesn't exist yet.

A solution is to use the real connection instead of the DummyConnection and to add a "read-only" flag to it. When starting the NewEntryWizard this flag must be set to true, so that no modifications are sent to the directory. When closing the Wizard this flag is resetted to its old value.