[GUI][Bug] Reset custom change address

[random-zebra]

First commit fixes a bug where the btnChangeAddress in SendWidget was deactivated, despite a custom address still being saved (e.g. saving a valid address, and then trying to change it to an invalid one).

Second commit addresses a feature request: being able to reset only the change address (without resetting the whole coin control) directly from within the dialog.
With this PR, when a custom change address is set, the text of the "CANCEL" button in SendChangeAddressDialog is changed to "RESET" and, when clicked, resets the default change address to CNoDestination (which means that a new change address will be created during the send operation).
The upright "X" button retains its original function of just closing the dialog.
Closes #1526

[Fuzzbawls]

I think we should take an extra precaution with regards to an invalid address being entered by resetting the change address. this will prevent a previously saved (valid) change address from persisting.

Index: src/qt/pivx/send.cpp
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- src/qt/pivx/send.cpp	(revision 4763c2c08d2f80b96027eda4ab4c2a7642d14e08)
+++ src/qt/pivx/send.cpp	(date 1587713628926)
@@ -601,6 +601,7 @@
                 ui->btnChangeAddress->setActive(true);
             } else {
                 inform(tr("Invalid change address"));
+                QMetaObject::invokeMethod(dialog, "reset", Qt::DirectConnection);
             }
         }
     }

[random-zebra]

Added a commit to move the address validation inside SendChangeAddressDialog.
The reason is twofold: first, it addresses the concern raised by @Fuzzbawls about a previously (valid) address being persisted when entering an invalid one. Second, it offers better UX, removing the need for an extra click, in case of errors during copy-paste.

[Fuzzbawls]

ACK b00b79c

[furszy]

Left a minor comment. Tested ACK b00b79c

[random-zebra]

Error message changed to "Invalid address" / squashed into last commit.

[furszy]

ACK 581fab6

[Fuzzbawls]

re-ACK 581fab6