Centralize standard LMMS plugin logo (#8124)

* Centralize standard LMMS plugin logo

Previously, every single plugin would have its own copy of this image.
Now, these plugins pull it from the theme. I'm not sure it's valuable
for this to be themeable, but AFAIK it's the place all the other
non-plugin resources are stored, so that's where it goes for now.

plugins/DynamicsProcessor/DynamicsProcessor.cpp

@@ -50,7 +50,7 @@ Plugin::Descriptor PLUGIN_EXPORT dynamicsprocessor_plugin_descriptor =
 	"Vesa Kivimäki <contact/dot/diizy/at/nbl/dot/fi>",
 	0x0100,
 	Plugin::Type::Effect,
-	new PluginPixmapLoader("logo"),
+	new PixmapLoader("lmms-plugin-logo"),
 	nullptr,
 	nullptr,
 } ;