Partial support of Aria bank XML support

[a13]

Hi. I tried to get Plogue sforzatron working using out-of-the-box SFZ files.

It didn't work as is, but my investigation led me to the issue

SFZ files use values like

default_path=$sample_dir/Cello/

where $sample_dir is defined inside the Plogue.sforzatron.bank.xml file

<Define name="$sample_dir" value="../Samples"/>

So when I replaced it with the value from the xml - everything worked just fine. I understand that to support the whole Aria stuff is impossible (given it's closed-source) and out of the project's scope - is it possible to add some rudimental <Define> parser?

[redtide]

It shouldn't be hard to implement and not that much out of scope if we support ARIA opcodes; why not an XML extension. I wonder how many instruments use this behavior.