Construct event channel outside of Context

This allows account manager to construct a single event channel and
inject it into all created contexts instead of aggregating events from
separate event emitters.

benches/receive_emails.rs

@@ -8,6 +8,7 @@ use deltachat::{
     context::Context,
     dc_receive_imf::dc_receive_imf,
     imex::{imex, ImexMode},
+    Events,
 };
 use tempfile::tempdir;
 
@@ -42,7 +43,9 @@ async fn create_context() -> Context {
     let dir = tempdir().unwrap();
     let dbfile = dir.path().join("db.sqlite");
     let id = 100;
-    let context = Context::new(dbfile.into(), id).await.unwrap();
+    let context = Context::new(dbfile.into(), id, Events::new())
+        .await
+        .unwrap();
 
     let backup: PathBuf = std::env::current_dir()
         .unwrap()