prepare: Set changetype=bech32 in BTC and LTC .conf files.

Rewrite .conf files to add changetype at startup if possible.
Add combine_non_segwit_prevouts function to coin interface.
Add option to list non-segwit UTXOs and combine_non_segwit_prevouts to gui.
Add test for changetype and combine_non_segwit_prevouts.

basicswap/interface/part.py

@@ -187,6 +187,9 @@ class PARTInterface(BTCInterface):
             ) + self.make_int(u["amount"], r=1)
         return unspent_addr
 
+    def combine_non_segwit_prevouts(self):
+        raise RuntimeError("No non-segwit outputs found.")
+
 
 class PARTInterfaceBlind(PARTInterface):