Also use Ignition GUI render event

[chapulina]

The render event was ported from ign-gazebo to ign-gui on gazebosim/gz-gui#68 but ign-gazebo isn't making use of the GUI event yet.

The plan is to:

• Emit both events on Citadel and Dome
• Deprecate ign-gazebo's render event on Edifice (see Deprecate some gazebo::gui events in favor of ign-gui events #595 )
• Remove ign-gazebo's render event on Ign-F

This PR accomplishes the 1st bullet point.

This way, users can write ign-gui plugins that work both with ignition::gui::Scene3D and ignition::gazebo::Scene3D.

[codecov]

Codecov Report

Merging #598 (4b1b1c0) into ign-gazebo3 (01fc2ff) will increase coverage by 12.00%.
The diff coverage is 86.66%.

@@               Coverage Diff                @@
##           ign-gazebo3     #598       +/-   ##
================================================
+ Coverage        65.71%   77.71%   +12.00%     
================================================
  Files              127      208       +81     
  Lines             6238    11595     +5357     
================================================
+ Hits              4099     9011     +4912     
- Misses            2139     2584      +445     

Impacted Files	Coverage Δ
include/ignition/gazebo/EntityComponentManager.hh	100.00% <ø> (ø)
include/ignition/gazebo/Link.hh	100.00% <ø> (ø)
include/ignition/gazebo/Model.hh	100.00% <ø> (ø)
include/ignition/gazebo/SdfEntityCreator.hh	100.00% <ø> (ø)
include/ignition/gazebo/Server.hh	100.00% <ø> (ø)
include/ignition/gazebo/ServerConfig.hh	100.00% <ø> (ø)
include/ignition/gazebo/System.hh	100.00% <ø> (ø)
include/ignition/gazebo/components/Component.hh	100.00% <ø> (ø)
include/ignition/gazebo/gui/GuiEvents.hh	0.00% <0.00%> (ø)
include/ignition/gazebo/gui/GuiRunner.hh	100.00% <ø> (+100.00%)	⬆️
... and 195 more

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