Add mouse shortcut for Fill and adapt docbook

Summary:
D14093 will add back middle-clicking to toggle between Fit (with
F as its keyboard shortcut) and 100% zoom. In addition,
D14094 sets ⇧+F as a shortcut for Fill.

Going along with this, here we add ⇧-middle-clicking for
Fill. The code is moved to DocumentView in order not to
duplicate the handling of the toggling behaviour.

We also adapt the docbook accordingly, as some users still seem to read
the manual.

CCBUG: 396360

Depends on D14093

Test Plan:
Middle-click multiple times on an image in View mode and observe
Fit is toggled. Repeat while holding down ⇧ for
Fill.
Proof-read docbook ("View mode" and "Tips" sections).

Reviewers: Gwenview, muhlenpfordt

Reviewed By: Gwenview, muhlenpfordt

Subscribers: muhlenpfordt, kde-doc-english

Tags: Documentation

Differential Revision: https://phabricator.kde.org/D14103