Maintain compatibility to Qt 5.12 (for Focal)

[rhaschke]

Fixes #391?

[codecov]

Codecov Report

Base: 42.10% // Head: 42.10% // No change to project coverage 👍

Coverage data is based on head (9074c5a) compared to base (2099b0c).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             ros2     #392   +/-   ##
=======================================
  Coverage   42.10%   42.10%           
=======================================
  Files          78       78           
  Lines        7787     7787           
=======================================
  Hits         3278     3278           
  Misses       4509     4509           

Impacted Files	Coverage Δ
...alization/motion_planning_tasks/src/task_panel.cpp	0.00% <ø> (ø)

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[quarkytale]

Worked for me!