[Places Item Edit Dialog] Fix accepting dialog with Return

[Places Item Edit Dialog] Fix accepting dialog with Return

By setting a parent for the QDialogButtonBox we let QDialog handle everything by itself.
Also, store the dialog button box as a member rather than just the OK button.

This removes the need to set the OK button as default manually (which didn't work anyway)
and fixes pressing Return while the input is focused erroneously opening the folder browser
rather than accepting the dialog.

REVIEW: 129213