qml PIN: do not lock inactive app, and remove timeout

- the activity callback does not work properly on android
  (does not work on my phone). Also, it duplicates the lock
  screen function of most phones.

- if we do not lock inactive app, then the PIN feature does
  not need a timeout, and is easier to understand without it.

- in Preferences, explain what it does

electrum/gui/qml/components/Preferences.qml

@@ -191,7 +191,7 @@ Pane {
                         }
                         Label {
                             Layout.fillWidth: true
-                            text: qsTr('PIN')
+                            text: qsTr('PIN protect payments')
                             wrapMode: Text.Wrap
                         }
                     }

electrum/gui/qml/components/main.qml

@@ -485,9 +485,6 @@ ApplicationWindow
         }
     }
 
-    property var _lastCorrectPinTime: 0
-    property int _pinValidSeconds: 5*60
-
     function handleAuthRequired(qtobject, method) {
         console.log('auth using method ' + method)
         if (method == 'wallet') {
@@ -513,14 +510,8 @@ ApplicationWindow
                 // no PIN configured
                 qtobject.authProceed()
             } else {
-                if (Date.now() - _lastCorrectPinTime <= _pinValidSeconds * 1000) {
-                    // correct pin entered recently, accept.
-                    qtobject.authProceed()
-                    return
-                }
                 var dialog = app.pinDialog.createObject(app, {mode: 'check', pincode: Config.pinCode})
                 dialog.accepted.connect(function() {
-                    _lastCorrectPinTime = Date.now()
                     qtobject.authProceed()
                     dialog.close()
                 })
@@ -538,27 +529,4 @@ ApplicationWindow
     property var _lastActive: 0 // record time of last activity
     property bool _lockDialogShown: false
 
-    onActiveChanged: {
-        console.log('app active = ' + active)
-        if (active) {
-            if (!_lastActive) {
-                _lastActive = Date.now()
-                return
-            }
-            // activated
-            if (Date.now() - _lastCorrectPinTime > _pinValidSeconds * 1000) {
-                if (_lockDialogShown || Config.pinCode == '')
-                    return
-                var dialog = app.pinDialog.createObject(app, {mode: 'check', canCancel: false, pincode: Config.pinCode})
-                dialog.accepted.connect(function() {
-                    _lastCorrectPinTime = Date.now()
-                    dialog.close()
-                    _lockDialogShown = false
-                })
-                dialog.open()
-                _lockDialogShown = true
-            }
-        }
-    }
-
 }