add qr-code-type for a webrtc-instance-pattern

introduce the qr-code-type `DCWEBRTC:[webrtc_instance]`.

dc_check_qr() returns this as the type DC_QR_WEBRTC
and these qr-codes can be passed to dc_set_config_from_qr() then;
this finally sets `webrtc_instance` using dc_set_config().

src/lot.rs

@@ -91,6 +91,9 @@ pub enum LotState {
     /// text1=domain
     QrAccount = 250,
 
+    /// text1=domain, text2=instance pattern
+    QrWebrtcInstance = 260,
+
     /// id=contact
     QrAddr = 320,