Adds TeleopPlugin with ignition-gui 3

[agalbachicar]

• Replicates UI from TeleopWidget which uses ign-gui0.
• Implements the same functionality.
• Missing automatic horizontal span.

teleop.mp4

To simplify reviewer's labor:

• Key events were migrated to TeleopPlugin.qml because it is just simpler to handle there.
• Button logic (change of label and driving flag) was migrated to TeleopPlugin.qml because it is just simpler to handle there.
• Almost all state attributes are QProperties, it is way simpler to share state and do bindings.

I did not intend to change behavior, if you find anything odd please let me know.

Part of #332