Add better support for Qt5 style slots

[kunaltyagi]

Closes #1742

Please check the proposed changes. I'll modify other properties once this is considered as an appropriate direction

API: adds ctor in properties ABI: no change, except as indicated by API

Description

Properties now have a ctor which allows providing normal C++ function pointers and lambda variables instead of Q_SLOTS

Checklist

• [x] Choose the proper target branch: latest release branch, for non-ABI-breaking changes, future release branch otherwise. Due to the lack of active maintainers, we cannot provide support for older release branches anymore.
• [ ] Did you change how RViz works? Added new functionality? Do not forget to update the tutorials and/or documentation on the ROS wiki
• [ ] While waiting for someone to review your request, please consider reviewing another open pull request to support the maintainers of RViz. Refer to the RViz Wiki for reviewing guidelines.

[kunaltyagi]

CI fails due to outstanding warnings. Most are from system headers

[rhaschke]

Sorry for the long delay on this. I didn't had time to look into that before. I noticed that your approach only supports lambdas, but not pointer-to-member functions as slots. See #1790 for an alternative approach.

[rhaschke]

@kunaltyagi, #1790 is ready for review and testing now. Closing here.