replace deprecated QGLWidget with QML

[Tatsuyuki Ishi]

As a part of Qt5 port, GL-based waveform renderers alongside with other things that use raw GL should be ported to the new API, QOpenGLWidget.

Some notes:
- The renderer GL code should be rewritten. They're in the deprecated GL API (glBegin, glEnd, glOrtho, ...) which should be replaced with modern OpenGL (see open.gl for some description).
- Either drop Qt4 or pollute the code with tons of preprocessor version checks.
- The OpenGL context can be shared without a helper class
- There's no guarantee that the OpenGL context is created upon construction, so such detection method is likely not to work

[Be (be.ing)]

Perhaps related: Bug #1718066

[Be (be.ing)]

https://github.com/mixxxdj/mixxx/pull/1863

[Daniel Schürmann (daschuer)]

This bug dies not describe a critical issue.
The issue described here is the deprecated state. That is not critical.

In general we should not rush into this mayor change for 2.2 it is simply not enough time for testing this mayor change.
If we cannot fix the Mac is issue without this change I vote for a qt4 release which is scheduled 2018-11-17 in three weeks.

[Be (be.ing)]

We have reports that Mixxx 2.2 is entirely unusable with macOS with QGLWidget, therefore this is critical: https://mixxx.zulipchat.com/#narrow/stream/109122-general/subject/MacOS.20rendering.20issue.20(possibly.20hiDPI.20related)/near/136167907

I'm not going to repeat myself why reverting to a library that hasn't been maintained in 7 years is not an acceptable solution.

[Be (be.ing)]

It seems this is related to building with the 10.14 macOS SDK. We are using the 10.13 macOS SDK on the build server for 2.2, so we may not need to rush this for 2.2.
https://bugreports.qt.io/browse/QTBUG-71292

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/8853