recognize t.me proxy qr codes (#5895)

this PR adds the type DC_QR_SOCKS5_PROXY to `dc_check_qr()` for
**supporting telegram proxy QR codes**. if returned, the UI should ask
the user if they want to us the proxy and call
`dc_set_config_from_qr();` afterwards (plus maybe `dc_configure()`).

idea is to improve our proxy story, follow ups may be:

- in UI, - move proxy out of "Account & Password", as a **separate
"Proxy Activity"** (it should stay in "Advanced" for now, however, below
"Server", which might be moved up)

- allow **opening the "Proxy Activity" from the welcome screens**
three-dot-menu (that would also solve a long standing issue that
entering the email address bypasses the proxy

- show proxy usage in the "Connectivity View" and/or add an **icon** to
the main chatlist screen (beside three-dot menu) in case some proxy is
in use; tapping this icon will open the "Proxy Activity"

- the the new "Proxy Activity", add a **share / show proxy QR code**
button. that would generate invite links in the form
`https://i.delta.chat/socks#...` - so that tapping then opens the app.
support for these links need to be added to core then.

- handle a list of proxies in core, offer selection in UI. the list
could be one for all profiles and could be filled eg. by normal invite
links or other channels

---------

Co-authored-by: iequidoo <117991069+iequidoo@users.noreply.github.com>

deltachat-ffi/deltachat.h

@@ -2507,6 +2507,7 @@ void            dc_stop_ongoing_process      (dc_context_t* context);
 #define         DC_QR_BACKUP                 251
 #define         DC_QR_BACKUP2                252
 #define         DC_QR_WEBRTC_INSTANCE        260 // text1=domain, text2=instance pattern
+#define         DC_QR_SOCKS5_PROXY           270 // text1=host, text2=port
 #define         DC_QR_ADDR                   320 // id=contact
 #define         DC_QR_TEXT                   330 // text1=text
 #define         DC_QR_URL                    332 // text1=URL
@@ -2560,6 +2561,10 @@ void            dc_stop_ongoing_process      (dc_context_t* context);
  *   ask the user if they want to use the given service for video chats;
  *   if so, call dc_set_config_from_qr().
  *
+ * - DC_QR_SOCKS5_PROXY with dc_lot_t::text1=host, dc_lot_t::text2=port:
+ *   ask the user if they want to use the given proxy and overwrite the previous one, if any.
+ *   if so, call dc_set_config_from_qr() and restart I/O.
+ *
  * - DC_QR_ADDR with dc_lot_t::id=Contact ID:
  *   e-mail address scanned, optionally, a draft message could be set in
  *   dc_lot_t::text1 in which case dc_lot_t::text1_meaning will be DC_TEXT1_DRAFT;