plugin-loader: refuse loading plugins with no metadata

[dkondor]

Fixes #2561

This is taking the second approach mentioned there, by not loading plugins that have no metadata (I assume that all plugins have an XML and it is loaded by wf-config, even if they have no options there).

[dkondor]

I did a bit more testing and realized that this only works if the plugin is completely "unknown".

However, if there is leftover configuration in wafire.ini, the crash from #2561 can still happen as apparently wf-config will create a section. Still, this fixes at least the case of completely newly installed plugins.

[soreau]

This is a NAK because plugins without metadata are valid to load. If metadata is installed or changes, wayfire must be restarted. If dynamic reloading of metadata is implemented, it would start in wf-config, as far as I understand. But then you would have to plumb it through the relevant wf-config consumers and it might be easier to just restart wayfire.

[dkondor]

Oh, I see, that does complicate things. I also feel that reloading metadata will have its own complications.
Another way to fix my original issue would be to dlopen() everything on startup in the plugin lib folder and later refuse to load plugins that are not already opened. Although this feels as a hack as well and could bring it's own problems. I might experiment with it if I have a bit more time.