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>

deltachat-ffi/deltachat.h

@@ -343,6 +343,12 @@ char*           dc_get_blobdir               (const dc_context_t* context);
  *                    and watch the `DeltaChat` folder for updates (default),
  *                    0=do not move chat-messages
  *                    changes require restarting IO by calling dc_stop_io() and then dc_start_io().
+ * - `only_fetch_mvbox`
+ *                  = 1=Do not fetch messages from folders other than the
+ *                      `DeltaChat` folder. Messages will still be fetched from the
+ *                       spam folder and `sendbox_watch` will also still be respected
+ *                       if enabled.
+ *                    0=watch all folders normally (default)
  * - `show_emails`  = DC_SHOW_EMAILS_OFF (0)=
  *                    show direct replies to chats only (default),
  *                    DC_SHOW_EMAILS_ACCEPTED_CONTACTS (1)=