Add debug logs to generateItems

It's always worth having logs for when state holders are created or destroyed (these are often the most interesting things happening in the application), so I thought it would be nice to have generateItems always log for you when it's doing that.

src/state/UserMedia.ts

@@ -130,6 +130,7 @@ export class UserMedia {
               // MediaViewModels don't support it though since they look for a unique
               // track for the given source. So generateItems here is a bit overkill.
               generateItems(
+                `${this.id} screenShares$`,
                 function* (p) {
                   if (p.isScreenShareEnabled)
                     yield {