Fix importing of bookmarks

Summary:
Also cleared some comments in BookmarkManager.h
Fixes BUG:316969
Related Google Code-In 2016 task:
https://codein.withgoogle.com/dashboard/task-instances/6308606150443008/

Reviewers: nienhueser, rahn

Subscribers: rahn, nienhueser

Differential Revision: https://phabricator.kde.org/D3989

BUG: 316969
FIXED-IN: Applications/17.04