Repo cleanup: reference one common LMMS default plugin logo

[rdrpenguin04]

In the PNG-to-SVG conversion pull requests, I notived that the LMMS logo SVG was basically being copied everywhere. While this is a definite improvement over the equivalent PNG being copied everywhere, it would be nice to have that default logo.svg at a common location that can either be implicitly used by default or easily referenced by all plugins.

[rubiefawn]

Not all the plugins use the LMMS logo (the LADSPA plugins, for example, use a penguin icon). What is happening here is that each plugin expects to have its own custom icon, but a bunch of them just happen to use the same one. I think deduplicating these is a good idea, but we must be careful to not break a plugin's ability to use its own custom icon.