Create GUI on demand

[osrf-migration]

Original report (archived issue) by Mabel Zhang (Bitbucket: mabelmzhang, GitHub: mabelzhang).

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Summary

Feature to create / change GUI on demand, and expose it in e.g. SdfEntityCreator->CreateEntities(sdf::Gui) along with the other entities.

Motivation

Logging playback currently must ignore <gui> tag in the recorded SDF file, since the GUI can only be created at startup. This results in behavior like https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-gazebo/pull-requests/181/acropolis-add-logging/activity#comment-94389783 , i.e. the recorded file uses <engine>ogre2</engine>, and the playback defaults to ogre, which results in the whole scene being a lot brighter.

[osrf-migration]

Original comment by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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• set component to "gui"