Tigger 1.6.0 release

This release is a major upgrade to the 1.4 / 1.5 series

Amongst others:

• It includes an upgrade to Qt5.x
• The GUI now has a dark theme
• The GUI now has dockable dialogs

All existing features you love have been fully ported to Python 3.x.

Porting brought to you by @razman786

v1.6 beta testing thanks to @razman786, @IanHeywood, @o-smirnov and @bennahugo