Changes

Python: deleted `src/plugins/thirdParty/Python/cmake/FindSQLite3.cmake`.
Indeed, a similar file is now part of CMake (as of version 3.14, which
is the minimum version of CMake that we need to build OpenCOR).

Python: we don't need QtQuickWidgets.

Python: renamed `Python Qt` to `PythonQt`.
This is how it is called...!

Python: some minor cleaning up with the use of our Python interface
file.

Python: replaced our CMake `EXCLUDE_PYTHON` definition with
`PYTHON_VERSION`.
To be consistent with our use of `CLI_VERSION`, which we have incidently
renamed to `GUI_VERSION`.