feat(dvk): delete a message — recording and label together

A deleted message is a free slot, per its operator; keeping the label made it
look half-deleted.

frontend/wailsjs/go/main/App.d.ts

@@ -160,6 +160,8 @@ export function CreateDatabase(arg1:string):Promise<void>;
 
 export function DVKCancelRecord():Promise<void>;
 
+export function DVKDelete(arg1:number):Promise<void>;
+
 export function DVKPlay(arg1:number):Promise<void>;
 
 export function DVKPreview(arg1:number):Promise<void>;