dropping support for Qt 5.6 and below

Qt 5.6 LTS support ends March 2019. Consequently, I think we should drop support for 5.6 in VTK too. Thoughts? Objections?


+1. Qt 5.9 required seems logical.

No objections from us. We used to build our AppImage in a CentOS 6 container, which only had Qt 5.6 via EPEL repo, but we’ve moved that to CentOS 7 for which there is Qt 5.9.

Here’s an MR updating it to Qt 5.9 since 5.9 is the oldest supported version at end of 03/2019.

Will need to update dashboards before this MR gets merged, will do that in coming days/weeks.

update, the MR has been merged. Qt 5.9 is the minimum supported Qt version in VTK master as of today.