gst-plugin: update so that PACKAGE definition is used

After building a plug in based off this template, I wanted to give it new a name. I tried to do it using the PACKAGE definition but it did not affect the name. This was of course because the name was explicitly written in the 'plugin_init' as well as in 'GST_PLUGIN_DEFINE'. In the former, it was written quoted, and in the latter it was written without quotes. I thought that this change would make it possible to change the name of the plugin in one single place.

I also think this points to a need to harmonize how 'GST_DEBUG_CATEGORY_INIT' and 'GST_PLUGIN_DEFINE' work, so that one could use the 'PACKAGE' defined in them both. From an API point of view, that would be great, but I see that it is complicated since GST_PLUGIN_DEFINE works differently.