Update wallet logic to account for viewing keys

The wallet code previously assumed that an unlocked wallet would always
have a spending key associated with a note decryptor. Viewing keys break
this assumption.

src/wallet/wallet.h

@@ -1121,7 +1121,11 @@ public:
     void SetBroadcastTransactions(bool broadcast) { fBroadcastTransactions = broadcast; }
     
     /* Find notes filtered by payment address, min depth, ability to spend */
-    void GetFilteredNotes(std::vector<CNotePlaintextEntry> & outEntries, std::string address, int minDepth=1, bool ignoreSpent=true);
+    void GetFilteredNotes(std::vector<CNotePlaintextEntry> & outEntries,
+                          std::string address,
+                          int minDepth=1,
+                          bool ignoreSpent=true,
+                          bool ignoreUnspendable=true);
     
 };