Replace VST3 category string with enum slice

I've been putting this off for a while, but now is finally the day. We
already did this for CLAP a while back. This is both simpler and less
error prone.

plugins/examples/gain/src/lib.rs

@@ -195,7 +195,8 @@ impl ClapPlugin for Gain {
 
 impl Vst3Plugin for Gain {
     const VST3_CLASS_ID: [u8; 16] = *b"GainMoistestPlug";
-    const VST3_CATEGORIES: &'static str = "Fx|Dynamics";
+    const VST3_SUBCATEGORIES: &'static [Vst3SubCategory] =
+        &[Vst3SubCategory::Fx, Vst3SubCategory::Tools];
 }
 
 nih_export_clap!(Gain);