gplugin/gplugin

Make GPluginManager a final type

2021-10-12, Gary Kramlich
a1b0fd07c7f1
Make GPluginManager a final type

* Use g_signal_new_class_handler to handle the accumulator based signals
* Remove the optional struct offset for the non accumulator based signals
* This all fixes the warnings from vapigen as well

Testing Done:
Ran the unit tests, gplugin-query, and gplugin-gtk3-viewer without issue.

Reviewed at https://reviews.imfreedom.org/r/1028/
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