Merge changes from origin/7.1

Merge changes from origin/7.1

0d4080b ContainerEventProvider returns style names from container. Fixes #10718
6e91bdf Add test for TransactionalPropertyWrapper memory leaks
f0aaf89 Fixed resetting of ComboBox if focused and new items allowed (#13413).
e033fcd Always initialize WebBrowser for new sessions (#13571)
168de1f Revert "Drag image for text-area should contain text of text-area (#13557)"
35e2a34 Fix FieldGroup and TransactionalPropertyWrapper memory leaks (#13438)
7e5d44d Introduce a drag threshold for Drag and Drop (#13381)
f227f0c Drag image for text-area should contain text of text-area (#13557).

Change-Id: Idb01471f8ab0c7118fa884c364e6bc200d13948a