Added screenshot to toolbar

[jennuine]

Closes gazebosim/gz-gui#95
Added a screenshot service which saves an image of the current 3D scene from the user camera.
This depends on the ign-gui screenshot plugin gazebosim/gz-gui#170

Depends on gazebosim/gz-gui#170

Updated gui.config toolbar layout to use x / y positions instead of anchors and to contain the screenshot plugin.

[codecov]

Codecov Report

Merging #588 (575bca4) into ign-gazebo3 (c9f815a) will increase coverage by 0.03%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-gazebo3     #588      +/-   ##
===============================================
+ Coverage        77.86%   77.90%   +0.03%     
===============================================
  Files              212      210       -2     
  Lines            11719    11648      -71     
===============================================
- Hits              9125     9074      -51     
+ Misses            2594     2574      -20     

Impacted Files	Coverage Δ
src/gui/Gui.cc	66.40% <0.00%> (-0.53%)	⬇️
src/Util.cc	95.15% <0.00%> (-0.03%)	⬇️
src/network/NetworkManagerPrimary.cc	78.89% <0.00%> (ø)
...ems/kinetic_energy_monitor/KineticEnergyMonitor.hh
...ems/kinetic_energy_monitor/KineticEnergyMonitor.cc
src/Server.cc	83.43% <0.00%> (+0.63%)	⬆️
.../gui/plugins/transform_control/TransformControl.cc	15.92% <0.00%> (+0.88%)	⬆️
src/gui/plugins/entity_tree/EntityTree.cc	13.73% <0.00%> (+3.90%)	⬆️

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[chapulina]

With the suggestions on gazebosim/gz-gui#171 and #598, the ign-gui plugin can be used with both ign-gazebo and ign-gui, so we don't need to duplicate the logic here.

If you think those changes are ok, I'd suggest we update this pull request to:

• Remove all the changes to Scene3D
• Add the screenshot plugin from ign-gui to gui.config next to the shapes on the grey toolbar.

[chapulina]

LGTM! We just need an ign-gui release before merging: gazebosim/gz-gui#188

[chapulina]

All the test failures seem to be due to a problem on Ignition Fuel's server, it's timing out:

$ ign fuel download -v 4 -u https://fuel.ignitionrobotics.org/1.0/OpenRobotics/models/Backpack
Downloading model: 
  Name: backpack
  Owner: openrobotics
  Server:
    URL: https://fuel.ignitionrobotics.org
    Version: 1.0

[Msg] Downloading model [https://fuel.ignitionrobotics.org/openrobotics/models/backpack]
[Err] [FuelClient.cc:596] Failed to download model.
  Server: https://fuel.ignitionrobotics.org
  Route: openrobotics/models/backpack/tip/backpack.zip
  REST response code: 504
Download failed.

I think we can ignore them and merge this.