Don't perform heavy computations on GUI Thread

[rgoulter]

Since we're using Qt5 anyway, might as well use its threading capabilities. https://doc.qt.io/qt-5.10/thread-basics.html

[rgoulter]

Examples of appropriate solutions:

If we want to show progress:

• Place the function in a reimplementation of QThread::run() and start the QThread. Emit signals to update progress.
• OR Place the function in a reimplementation of QRunnable::run() and add the QRunnable to a QThreadPool. Write to a thread-safe variable to update progress.
• OR Run the function using QtConcurrent::run(). Write to a thread-safe variable to update progress.

If we don't care about the progress:

• Run the function using QtConcurrent::run(). Have a QFutureWatcher emit the finished() signal when the function has returned, and call QFutureWatcher::result() to get the function's return value.

[rgoulter]

TBH I feel showing 'progress' would be quite useful. e.g. our Delaunay algorithm is very slow. I kinda wanna see how slow.

Also, if we show 'progress'; it may be possible to "show" the current state of the algorithm? (Or, maybe that could be a separate issue after this).