[SDDM Theme] Select all and focus password field on error

When logging in failed, focus the password field and select all, so you can start typing a new password thereafter.
I did the same in the lock screen.

There's a workaround for a bug which draws focus away from the password field on login
but never put it back if logging in failed.

Differential Revision: https://phabricator.kde.org/D2767

sddm-theme/Login.qml

@@ -62,6 +62,14 @@ SessionManagementScreen {
                 event.accepted = true
             }
         }
+
+        Connections {
+            target: sddm
+            onLoginFailed: {
+                passwordBox.selectAll()
+                passwordBox.forceActiveFocus()
+            }
+        }
     }
     PlasmaComponents.Button {
         id: loginButton