New Apr2019 stable branch created

I have created a new stable branch for the next feature release. This branch was created in order to stabilize the code before the feature release.

From now on *only* bug fixes may go to this new stable branch. I reserve the right to undo any changes that do not comply with this rule. I also reserve the right to admit changes that are not strictly bug fixes.

The name for the branch is Apr2019. So in each of your stable checkout directories you can do hg pull hg update -rApr2019 to update to this new branch.