Add cancel button to rescan progress dialog

Adds a cancel button to the rescan progress dialog. When it is clicked,
AbortRescan is called to abort a rescan

src/interfaces/wallet.cpp

@@ -132,6 +132,7 @@ public:
     {
         return m_wallet.ChangeWalletPassphrase(old_wallet_passphrase, new_wallet_passphrase);
     }
+    void abortRescan() override { m_wallet.AbortRescan(); }
     bool backupWallet(const std::string& filename) override { return m_wallet.BackupWallet(filename); }
     std::string getWalletName() override { return m_wallet.GetName(); }
     bool getKeyFromPool(bool internal, CPubKey& pub_key) override

src/interfaces/wallet.h

@@ -65,6 +65,9 @@ public:
     virtual bool changeWalletPassphrase(const SecureString& old_wallet_passphrase,
         const SecureString& new_wallet_passphrase) = 0;
 
+    //! Abort a rescan.
+    virtual void abortRescan() = 0;
+
     //! Back up wallet.
     virtual bool backupWallet(const std::string& filename) = 0;