Export Cantor::PluginPanel and Cantor::PluginPanelHandler and minor improvments

1 job for master in 6 minutes and 18 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #59045
linux kf5-qt5 qt5.12


Name Stage Failure
linux kf5-qt5 qt5.12 Build
  cmake_install.cmake:74 (include)

make: *** [Makefile:106: install] Error 1
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ERROR: Job failed: exit code 1