.rdf files support for LADSPA

[jpka-]

Hello.
Please add support for reading .rdf files (/usr/share/ladspa/rdf/) in parallel with .so ones for LADSPA plugins. I need rdf:about field. There is data from .so files extracted now, but they are not contains rdf:about value. Example: https://archlinux.org/packages/extra/x86_64/caps/ installs, along .so, /usr/share/ladspa/rdf/caps.rdf which have some rdf:about's like 'http://quitte.de/dsp/caps.html#Noisegate', this useful data is lost currently, afaik.
If can be implemented, this value is good to be concatenated to (or replaced for, if we wish it to match for .lv2 behavior) 'Label/URI' field of plugin edit (properties).
If can be implemented, and string passed to 'carla_widgets.py', i will add LADSPA support to newly introduced 'Description' tab, as per #1961.
Thanks.

[jpka-]

There are another useful fields within .rdf, like preset names in form of <ladspa:Point rdf:value="0" ladspa:hasLabel="AfterBurn" />... @ /usr/share/ladspa/rdf/tap_reverb.rdf , this info also lost currently. Is it possible to have it at Carla Rack present at top of panel as drop-down list, like MVerb's presets, please?
P.S. tap_reverb is from from tap-plugins Arch AUR.

[jpka-]

Related: #156