expose drum-filter settings in LV2
brush up leslie LV2

* add horn/drum level control
* add drum filter (to extended version)
* expose RPM (in extended version)
clean up LV2 RDF
update button image
B3 LV2 GUI: fix possible mem-leak (empty dir)
don't include blender and gimp images in release

They're are only used to bootstrap look&feel, the actual
mesh and texture which were created from them were
tweaked and modified manually.