Add a plug-in tab in the preferences.
Each plug-in is listed, with a “Enabled” parameter and a “Zoom support”
parameter.

By default all plug-ins are enabled and all plug-ins zoom support is
set to the plug-in default.
However some are filtered out in the following functions in
Settings.cpp:

filterDefaultActivatedPlugin
filterDefaultRenderScaleSupportPlugin

Add plug-ins there to change the default value for these plug-ins.