qml: don't initialize instance variables on class scope for non-singletons

(this somehow escaped attention before, as most objects usually don't have multiple instances,
unless multiple wallets are open at the same time.)
Also, move all signal declarations, class constants and variables to the top of class definitions.

electrum/gui/qml/qeinvoice.py

@@ -44,14 +44,14 @@ class QEInvoice(QObject):
 
     _logger = get_logger(__name__)
 
-    _wallet = None
-    _canSave = False
-    _canPay = False
-    _key = None
-
     def __init__(self, parent=None):
         super().__init__(parent)
 
+        self._wallet = None
+        self._canSave = False
+        self._canPay = False
+        self._key = None
+
     walletChanged = pyqtSignal()
     @pyqtProperty(QEWallet, notify=walletChanged)
     def wallet(self):
@@ -117,15 +117,8 @@ class QEInvoice(QObject):
         return self._wallet.wallet.lnworker.num_sats_can_send()
 
 class QEInvoiceParser(QEInvoice):
-
     _logger = get_logger(__name__)
 
-    _invoiceType = QEInvoice.Type.Invalid
-    _recipient = ''
-    _effectiveInvoice = None
-    _amount = QEAmount()
-    _userinfo = ''
-
     invoiceChanged = pyqtSignal()
     invoiceSaved = pyqtSignal([str], arguments=['key'])
 
@@ -140,6 +133,13 @@ class QEInvoiceParser(QEInvoice):
 
     def __init__(self, parent=None):
         super().__init__(parent)
+
+        self._invoiceType = QEInvoice.Type.Invalid
+        self._recipient = ''
+        self._effectiveInvoice = None
+        self._amount = QEAmount()
+        self._userinfo = ''
+
         self.clear()
 
     @pyqtProperty(int, notify=invoiceChanged)