[QGIS-Developer] QGIS 4.0 - let's start some early discussions!

[Nyall Dawson]

Hi all,

Following conversation from https://github.com/qgis/QGIS/pull/30234, I
think it would be beneficial if we start the conversation happening
about QGIS 4.0, and to throw some thoughts about timelines out there.

**Before anyone panics -- I'm expecting multi-year time frames here!
But I think we should START this discussion, and have at least some
idea of the time frames we're all aiming toward for a future 4.0
release.**

For reference: Qt upstream has previously hinted at November 2020 for
Qt 6, at which stage support for 5.x will be dropped. Discussions so
far are moving toward Qt 6 being a "gentle" cleaning, so there's
likely (hopefully?) not a lot we'll be forced to do to adapt to this.

I think we should aim for a similar goal -- a "gentle" API break, as
opposed to the huge-clean-and-break-everything-we-possibly-can
approach we took for 3.x (with good reason!).

Thoughts?

Nyall