CSV Data Store plugin: prevent people from un/selecting it [ci skip].

This will remain the case until we have implemented issue #451.
Merge branch 'master' of github.com:opencor/opencor
Slight update to the Windows distribution README.txt file [ci skip].
Slight update to the developer documentation [ci skip].
Some cleaning up in our batch scripts [ci skip].
Some work on making it possible to build both the 32-bit and 64-bit versions of OpenCOR on the same 64-bit Windows machine [ci skip].
Some minor cleaning up [ci skip].
Done making it possible to build both the 32-bit and 64-bit versions of OpenCOR on the same 64-bit Windows machine [ci skip].
Renamed the DataStoreVariable's 'name' property to 'label'.
Renamed the Core::DockWidget class to Core::WindowWidget.
Moved the Spinner widget to the Core plugin.

Indeed, anything that is needed by something from the Core plugin
really ought to be in the Core plugin since the Core plugin shouldn’t
have any dependency whatsoever.
Some minor cleaning up [ci skip].
Added a spinner widget to our window widget (#452) [ci skip].
Renamed the Core::DockWidget class to Core::WindowWidget.
We now have a spinner widget in our 'core' view and window widget classes [ci skip].
Some minor cleaning up [ci skip].
CellML Annotation view: allow all variables to be exposed, not only 'defined' ones (closes 458).
Travis-CI: build using one core only.

Until now, we were using half of the cores available on a Travis-CI
build machine, but this resulted in regular g++ crashes on 64-bit
Ubuntu. So, now, it’s going to take longer to build, but at least it
should be fine…
Some minor cleaning up [ci skip].
Some reworking of our spinner support widget [ci skip].

It now only has a spinner widget when really necessary, i.e. when one
needs to be shown.