Migrate to Python3, QT5 and pyside2

[ndevenish]

This is a placeholder pull request for progress towards upgrading DUI for CCP4's qt5/pyside2/python3 distribution.

As part of this work, there are no constraints on campsite improvements/refactoring, so there are many more changes that are required for a theoretical minimally invasive upgrade, and other tickets being fixed along the way.

Currently fixed associated issues:

• 77: Add Diamond and CCP4 logo

• 79: Not using monospace text for log on mac(tested)/windows(tbd)

• 84: Cannot import filenames containing spaces

• 106: Layout tidying and cross-platform support: Fusion removes problem.

• 107: "Reset" button cropped

• 110: Mac retina icon size issues: QT5 fusion unified layout should resolve this

• 127: Flashing windows on open

• 164: Image filename not reloaded to import page on windows

• 166: Opening DUI on reindex step opens reindex window

Other changes:

• Don't create a dui_files/ folder until a command is run - doesn't leave an empty one if nothing is done
• Reindex dialog:
  • Size reindex dialog to the table, it always showed too small before.
  • Don't reindex if the user selected the same result as before
  • Show a cancel button when reopening on an existing node
  • Select the previous solution when re-reindexing
• General docstrings, light refactoring, fixes on way to other tasks
• Fix alignment of left tree and tabs; vs

Untested issues that should be resolved by the move:

• 109: Upgrading to the QT5 Web viewer should fix this