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Remove support for seed-based Javascript plugins

The seed bindings lack subclassing support and as such
are holding back the capabilities that libpeas can depend
on for certain features that plugins are expected to use.
The bindings are also rather unmaintained these days.

https://bugzilla.gnome.org/show_bug.cgi?id=739306
Remove everything the seed plugin loader used

This is no longer needed as the seed plugin
loader has been removed.

https://bugzilla.gnome.org/show_bug.cgi?id=739306
Rename helpers to utils
Update URLs for gnome.org changes
Cleanup PeasPluginInfo coding style and docs
No longer use a GHashTable for storing the plugin loaders

Replace with a static array and functions to convert
from a loader name to an id and back. Also, warn if an
unknown loader is given to peas_engine_enable_loader()
or specified in a .plugin file.