Sensor topics available through components and GUI

[chapulina]

Addresses #85
Requires gazebosim/gz-sensors#33 and gazebosim/gz-transport#153

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While working on #239, I thought it would be good to also address #85. This adds a SensorTopic component that can be checked from any system and is also displayed on the GUI. It uses the new API on ign-sensors to get the real topic that is being published, after all inputs and modifications have been applied to it.

⚠️ This PR doesn't add topic components for rendering sensors because the API on this branch doesn't make it easy to access the ECM and the sensor pointer at the same time. The Sensors::CreateSensor function on Dome takes an Entity ID which will make it easier to add topics for those sensors, so my current suggestion is to only support topic components for rendering sensors from Dome. Let me know what you think, @iche033 .

[iche033]

The Sensors::CreateSensor function on Dome takes an Entity ID which will make it easier to add topics for those sensors, so my current suggestion is to only support topic components for rendering sensors from Dome.

sounds good to me.