Sort plugin list in alphabetical order (including when filtering)

[mabelzhang]

Signed-off-by: Mabel Zhang mabel@openrobotics.org

🎉 New feature

Closes gazebosim/gz-sim#175
Follows up on #277

Summary

Sort plugin menu in alphabetical order by overriding lessThan, the default defined in include/ignition/gui/qml/IgnSortFilterModel.qml.

Test it

ign gui

Click on the three vertical dots in the upper-right corner for the plugin menu.

Before (not sorted):

After (sorted alphabetically):

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #387 (bfc4c04) into ign-gui3 (5a02f30) will decrease coverage by 28.44%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           ign-gui3     #387       +/-   ##
=============================================
- Coverage     59.57%   31.12%   -28.45%     
=============================================
  Files            20       29        +9     
  Lines          2471     1343     -1128     
=============================================
- Hits           1472      418     -1054     
+ Misses          999      925       -74     

Impacted Files	Coverage Δ
include/ignition/gui/GuiEvents.hh
include/ignition/gui/Helpers.hh
include/ignition/gui/Plugin.hh
src/Application.cc
src/Conversions.cc
src/Helpers.cc
src/MainWindow.cc
src/Plugin.cc
src/ign.cc
src/plugins/image_display/ImageDisplay.cc
... and 39 more

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