Build Mantid with Qt5 only

[AndreiSavici]

This is an umbrella ticket for building on systems where there is no Qt4 (Ubuntu 20.04). It should prepare for the removal of MantidPlot and supporting code but not remove that yet.

• Build Mantid PR # Build Mantid and unit tests #28651
• Build tests PR Build Mantid and unit tests #28651
• Disable Qt4 tests if -DENABLE_MANTIDPLOT=OFF Qt4 tests on ubuntu 20.04 #29007 , PR Unit tests ununtu 20 04 #29013
• UnitTests pass Ensure UnitTests pass when only workbench and Qt5 is built #29390
• Packaging on macOS without MantidPlot (Cannot properly package on OSX with only ENABLE_WORKBENCH and not ENABLE_MANTIDPLOT #28833)
• Build documentation/doc tests Build documentation with workbench only #28647, PR Build documentation in vanilla Python instead of MantidPlot #29684, Failing doc tests on Ubuntu 20.04 #29733
• Switch to use workbench bundle in DMGInstaller in mantidinstaller. This will switch the system test CI jobs to use workbench on macOS. Remove the QT_API=pyqt4 injected variable in master_systemtests-osx and release_systemtest-osx PR Switch macOS system test runner to use Python from MantidWorkbench bundle #29749
• Inject QT_API=pyqt5 on rhel7 wrapper script PR Startup scripts now default to pyqt5 #29921
• Once workbench only can be packaged disable MantidPlot/Vates in build scripts.

The outcome of this is that a build with -DENABLE_MANTIDPLOT=OFF should produce a working package with workbench only.

Edited (mgigg: moved to issue #30268)
~~Nice to haves but not required to be able to disable MantidPlot:

• [ ] Systemtests pass System tests pass on Ubuntu 20.04 #29362 This is not technically required for Qt5 only but these tests fail on Ubuntu 20.04
• [ ] Remove code in #if QT_VERSION < QT_VERSION_CHECK(5, 0, 0) directives
• [ ] Remove Qt4 packages from RPM/DEB dependencies
• [ ] Remove Qt4 from developer packages

Development docker images will need to have Qt4 installed until we decide to disable MantidPlot on the builds but then they can be removed.

[AndreiSavici]

With changes in #28651, most of the failing tests are because we force Qt4 testing even if it's not available.

[martyngigg]

@AndreiSavici The system test failures on 20.04, with the exception of PVPython, are not all required to be fixed for us to disable MantidPlot on the other platforms.
Would you be happy if we sorted out disabling MantidPlot first? I'm just aware that Release 6.0 is coming and the code freeze is the start of January so getting this is donesooner is probably better.

[AndreiSavici]

@martyngigg Fine with me. The ORNL team is looking into those tests anyway, but it can be done after removing MantidPlot

[martyngigg]

Brilliant. Thanks for the update.

[AndreiSavici]

@martyngigg When the PR with removing MantidPlot is merged, we need to send an email to the user community to say that mantidplotnightly is gone.

[martyngigg]

I've moved the remaining parts of this to a new issue that deals with actually removing MantidPlot, #30268, so I'll close this now.