[PDI-6932] - provide comparator to sort by date (not string)

add comparatorBinding to xul markup
implement simple comparator for UIRepositoryObject
add unit test for comparator
after several attempts gave up idea tyo implement Xul UI unit test
simulating the entire (mis)behavior:
to many issues with Xul SWT implemantation
Merge pull request #114 from mtseu/pdi-6932

Pdi 6932 - enable date-sorting in repository explorer