Upgrade to Tycho 4.0

Tycho 3

Once Tycho 3.0 is released, we'll want to upgrade to it at some time.

Then we should check how that relates to SLF4J warnings we get:

• #343 (comment 679135)
• https://github.com/eclipse/tycho/issues/874

Tycho 4

There is a bug in Tycho 3 that prevents upgrading to it, as it makes our build fail on Jenkins. We'll need to migrate to Tycho 4 instead.