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/qeaddressdetails.py

@@ -8,27 +8,27 @@ from .qewallet import QEWallet
 
 
 class QEAddressDetails(QObject):
+    _logger = get_logger(__name__)
+
+    detailsChanged = pyqtSignal()
+
     def __init__(self, parent=None):
         super().__init__(parent)
 
-    _logger = get_logger(__name__)
+        self._wallet = None
+        self._address = None
 
-    _wallet = None
-    _address = None
+        self._label = None
+        self._frozen = False
+        self._scriptType = None
+        self._status = None
+        self._balance = QEAmount()
+        self._pubkeys = None
+        self._privkey = None
+        self._derivationPath = None
+        self._numtx = 0
 
-    _label = None
-    _frozen = False
-    _scriptType = None
-    _status = None
-    _balance = QEAmount()
-    _pubkeys = None
-    _privkey = None
-    _derivationPath = None
-    _numtx = 0
-
-    _historyModel = None
-
-    detailsChanged = pyqtSignal()
+        self._historyModel = None
 
     walletChanged = pyqtSignal()
     @pyqtProperty(QEWallet, notify=walletChanged)