Some minor cleaning up [ci skip].
Revert "Some minor cleaning up [ci skip]."

This reverts commit 5c22c16917e7b9d7c6885ccba09c6443043d2c1f.
Rely on JSON files to download the different versions/snapshots of OpenCOR (closes #505).
Removed some Qt libraries, which are not needed anymore by plugins on OS X.

Probably the result of some Qt update, on which I didn’t pick up…
Some minor cleaning up [ci skip].
Rely on JSON files to download the different versions/snapshots of OpenCOR (closes #505).

Those are not pure JSON files anymore, but JavaScript files that
contain some JSON. Indeed, to have pure JSON files prevented us from
testing the download pages locally. The only way to test them was
through a web server, which was exactly what we want to avoid (see
issue #490). The current solution still allows us to do what we need
for issue #1, so we are fine. It’s just not as ‘neat’…
Some minor cleaning up [ci skip].
Some minor cleaning up [ci skip].
Rely on JSON files to show what is new in OpenCOR (closes #506).
Some work on the update feature (#1) [ci skip].
CellML Annotation view: handle possible SSL errors [ci skip].
Documentation: slight update to the developer documentation [ci skip].
Some minor cleaning up [ci skip].
CellML Annotation view: handle possible SSL errors [ci skip].
Some minor cleaning up [ci skip].
Main executable: some work towards removing dependencies on some of the Core plugin features (#508) [ci skip].
CellML export to user-specified format: updated the Python format [ci skip].

See https://github.com/cellmlapi/cellml-api/pull/7.
Some work on check for updates feature (#1) [ci skip].
Some work on check for updates feature (#1) [ci skip].
Main executable: removed dependencies on some of the Core plugin features (closes #508).