Fixed few bugs related to login/logout

* Pressing the logout button now brings the setup page up again
* Removing a server and readding one of which the hash of the address
  has a higher value than the previous one does no longer cause the
  application to request a login each time you open it.
* The method for deciding if the user is in setup has been changed. It
  now checks if a page in the pageStack has the property _isSetupPage,
  which works better than manually keeping track of state.

qml/pages/setup/AddServerPage.qml

@@ -14,6 +14,7 @@ Dialog {
 	readonly property string address: serverSelect.currentItem._address
 	readonly property bool addressCorrect: serverSelect.currentIndex > 0 || manualAddress.acceptableInput
 	readonly property string serverName: serverSelect.currentItem._name
+    readonly property bool _isSetupPage: true
 	
 	
 	acceptDestination: AddServerConnectingPage {

qml/pages/setup/LoginDialog.qml

@@ -35,7 +35,7 @@ Dialog {
 			onAuthenticatedChanged: {
 				if (ApiClient.authenticated) {
 					console.log("authenticated!")
-                    pageStack.replaceAbove(pageStack.previousPage(firstPage), Qt.resolvedUrl("../MainPage.qml"))
+                    pageStack.replaceAbove(null, Qt.resolvedUrl("../MainPage.qml"))
 				}
 			}
 			onAuthenticationError: {