Do not lose translation of Places items if a Dolphin window is closed

If the "Places" are updated outside the current Dolphin process,
PlacesItemModel::updateBookmarks() is called, which invokes
PlacesItem::setBookmark(const KBookmark& bookmark) for each item. This
commit ensures that the correct translation is used for system
bookmarks like "Root", "Network", etc.

Without this patch, all Places entries in the remaining Dolphin windows
would switch to US English if one of multiple windows is closed.

See http://lists.kde.org/?t=139660468400004&r=1&w=2 for details.

BUG: 334999
REVIEW: 118207
FIXED-IN: 4.13.2