Dvg-pyqtgraph-threadsafe

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* Neater mechanism to support both PyQt and PySide
* Added benchmarks
* Improved demos

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* Added support for PyQt5, PyQt6, PySide2 and PySide6 as suggested via a pull
request by Mathijs van Gorcum (https://github.com/mvgorcum).

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* Removed redundant argument ``capacity`` from init ``PlotCurve()``. It is
not using a ringbuffer and, hence, does not need a capacity.

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Bug-fixes:

* The use of ``typing.TypedDict`` broke support under Python 3.6
and 3.7. Fixed by conditional import ``typing_extensions``.
* Curve plotting was broken when ``setClipToView(True)`` and the curve
data extended past the viewbox limits, when not using OpenGL. The cause was
my incorrect calculation of ``connect``. Fixed by commenting out ``connect``
again. Curves will now show continuously (linear interpolation) whenever a NaN
is encountered, instead of as correctly fragmented. That's acceptable.

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* Renamed parameter ``LegendSelect.curves`` to
``LegendSelect.linked_curves``
* Changed base of class ``LegendSelect()`` from ``QWidget`` to ``QObject``
* Added class ``PlotManager()``

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* Workaround: ``PyQt5 >= 5.12.3`` causes a bug in ``PyQtGraph`` where a
curve won't render if it contains NaNs (but only in the case when OpenGL is
disabled). The curve will now be displayed correctly, i.e., fragmented
whenever a NaN is encountered. When OpenGL is enabled, linear interpolation
will occur at the gaps as per ``pyqtgraph.plotCurveItem.paintGL()``.