Custom CMake targets are not shown in CMake build step settings

It was noticed while preparing for new round of Qt for MCUs training.

When importing one of the examples from Qt for MCUs 1.8.0 (`examples/translation`) we were adding additional CMake build step for triggering `update_translations' target (this is automatically created for applications that use translated strings). In Qt Creator 4.15.0 this target is missing from the list and it is impossible to enable that. Previously, when using Qt Creator 4.13.3 this was working as expected.

The `update_translations` target is created using `add_custom_target` CMake function and as such is marked as utility target. So far it seems that it is filtered out (see commit 
3da99f783296f32051b36e12b4d17bba76db4cb4).
 
If this is expected behaviour, I would like to have a checkbox like "Show all targets" to avoid unnecessary confusion. Right now it gives impression that Qt Creator is not able to detect some targets and there are lots of scenarios for larger projects to create such targets for number of custom tasks.