Prevent and warn when trying to enable multiple loaders

Using multiple plugin loaders will cause memory leaks due to
both the python 2/3 and seed binding using toggle refs. Also
mixing python version will never work.
Chain-up to finalize in the peas plugin loader
Simplify installing gettext in the python plugin loader

Also check for an error after installing.
Fix error message when popping log hooks fails
Have silent subprocess tests inherit stderr for easier debugging

For the engine shutdown test we cannot inherit stderr as it
prints to stderr.
Be more explicit about when a plugin loader is not enabled

Also silence a warning when we have already warned.
Do not test the properties for Python extensions

Because Python extensions are no longer wrapped testing
the properties is just pointless.
Remove INSTALL file
Silence an ImportError to stderr in the Python tests