Flub/watch mvbox only (#3028)

* Make set_config() look a bit nicer

* Add OnlyFetchMvbox option

* Add test for the config

* Add option to only watch mvbox

This is supposed to support having a server-side rule which moves
emails to the mvbox already.  The new option makes sure the mvbox is
wathched and also makes sure no messages are feched from folders other
than the mvbox and the spam folder if enabled.  It does not interact
with the other settings.

* Fixup ignore conditions

* Cleanup some bits

* Watch the mvbox when `WatchMvboxOnly` is set

* Rename back to only_fetch_mvbox (flub said it's OK for him)

* typo

* clippy, more typos

Co-authored-by: Hocuri <hocuri@gmx.de>

src/config.rs

@@ -74,6 +74,13 @@ pub enum Config {
     #[strum(props(default = "0"))]
     SentboxMove, // If `MvboxMove` is true, this config is ignored. Currently only used in tests.
 
+    /// Watch for new messages in the "Mvbox" (aka DeltaChat folder) only.
+    ///
+    /// This will not entirely disable other folders, e.g. the spam folder will also still
+    /// be watched for new messages.
+    #[strum(props(default = "0"))]
+    OnlyFetchMvbox,
+
     #[strum(props(default = "0"))] // also change ShowEmails.default() on changes
     ShowEmails,
 
@@ -225,6 +232,11 @@ impl Context {
         Ok(self.get_config_int(key).await? != 0)
     }
 
+    pub(crate) async fn should_watch_mvbox(&self) -> Result<bool> {
+        Ok(self.get_config_bool(Config::MvboxMove).await?
+            || self.get_config_bool(Config::OnlyFetchMvbox).await?)
+    }
+
     /// Gets configured "delete_server_after" value.
     ///
     /// `None` means never delete the message, `Some(0)` means delete
@@ -281,31 +293,25 @@ impl Context {
                     }
                 }
                 self.emit_event(EventType::SelfavatarChanged);
-                Ok(())
             }
             Config::DeleteDeviceAfter => {
-                let ret = self
-                    .sql
-                    .set_raw_config(key, value)
-                    .await
-                    .map_err(Into::into);
+                let ret = self.sql.set_raw_config(key, value).await;
                 // Force chatlist reload to delete old messages immediately.
                 self.emit_event(EventType::MsgsChanged {
                     msg_id: MsgId::new(0),
                     chat_id: ChatId::new(0),
                 });
-                ret
+                ret?
             }
             Config::Displayname => {
                 let value = value.map(improve_single_line_input);
                 self.sql.set_raw_config(key, value.as_deref()).await?;
-                Ok(())
             }
             _ => {
                 self.sql.set_raw_config(key, value).await?;
-                Ok(())
             }
         }
+        Ok(())
     }
 
     pub async fn set_config_bool(&self, key: Config, value: bool) -> Result<()> {