Move each plugin to its own directory

[chapulina]

Builds on top of #63

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We were not installing plugin headers, so there's no need for them to be in the include folder.

This pull request also moves each plugin into its own directory so the code is more organized and we can more cleanly add plugins with multiple files.

The new ign_gui_add_plugin function helps reduce the boilerplate for each plugin.

Other changes:

• Removed QWT code that was commented out - we won't need that with QtQuick.
• Removed the Requester and Responder cc files since those plugins were not being compiled.

CC @claireyywang and @AmrElsersy

[codecov]

Codecov Report

Merging #64 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   14.56%   14.56%           
=======================================
  Files          13       13           
  Lines        1332     1332           
=======================================
  Hits          194      194           
  Misses       1138     1138           

Impacted Files	Coverage Δ
src/plugins/world_control/WorldControl.cc	1.16% <ø> (ø)
src/plugins/world_stats/WorldStats.cc	1.12% <ø> (ø)

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[claireyywang]

Wow this is so neat! much easier to understand the structure now 🤩 I still need to try it locally, but seems like the CIs are failing. Missing dependency in CMakeLists.txt?

1: [GUI] [Err] [Application.cc:369] Failed to load plugin [TestBadInheritancePlugin] : couldn't get [ignition::gui::Plugin] interface.
1: [GUI] [Err] [Application.cc:333] Failed to load plugin [TestNotRegisteredPlugin] : couldn't load library on path [/var/lib/jenkins/workspace/ignition_gui-ci-pr_any-ubuntu_auto-amd64/build/lib/libTestNotRegisteredPlugin.so].

clicked the wrong button

[claireyywang]

LGTM! thanks for refactoring!

[chapulina]

but seems like the CIs are failing

Those error messages are expected, because those tests are making sure that we properly handle invalid plugins and don't crash. I see some builds list these lines as "problems" though, so I ticketed an issue for that: gazebo-tooling/release-tools#205

[claireyywang]

but seems like the CIs are failing

Those error messages are expected, because those tests are making sure that we properly handle invalid plugins and don't crash. I see some builds list these lines as "problems" though, so I ticketed an issue for that: ignition-tooling/release-tools#205

I see. I was able to build locally with no error -> approval