Enable redo for undone image operations

Summary:
Redoing an undone image operation (e.g. crop, rotate) does not work.
This is caused by a missing redo() override in the derived
QUndoCommand class.
The patch adds the redo() and moves/adjusts some calls to make
undo/redo work properly.
Additionally adds an autotest for undo/redo in DocumentTest.

FIXED-IN: 18.04.2

Test Plan:

• Use different image operations to edit one or more image(s)
• Undo/redo operations

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: huoni, rkflx

Differential Revision: https://phabricator.kde.org/D12105