Fixed ref constraints handling:

dest tables are ordered according to the master-details relationships - independent tables first then tables that depends on them only and so on.
Loops created by foreign keys are detected and corresponding FK's are disabled before data pump and enabled after.
DDL statement to enable (create) FK is fixed for the case when master table is distinct from detail one.
Table nodes in dest treeview are correctly marked as loop parts. Also such nodes are expanded and nodes of 'to be disabled' FK's are underlined.
It worked for self referenced tables only, now loops of any number of participant tables is detected correctly.