Introduce interactive test for panels

The purpose of this interactive test is to be able to
prototype new features for CcPanel without having to
adapt existing panels.

Three test panels were added as part of this initial
patch:

 * Dynamic panels: panel exercising the ability to
   show and hide itself at runtime.
 * Header: panel that embeds a header widget.
 * Static Initializer: panel that sets a static init
   function to run at startup.
11 jobs for gbsneto/interactive-test-panels in 3 minutes and 19 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #144978
build

00:01:54

 
  Test
passed #144980
asan

00:01:24

passed #144979
test

00:01:11

 
  Manual
manual #144985
allowed to fail manual
armv8
manual #144981
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flatpak
manual #144984
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msan
manual #144986
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ppc64le
manual #144982
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tsan
manual #144983
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ubsan
 
  Review
manual #144987
allowed to fail manual
review
manual #144988
allowed to fail manual
stop_review