Add GPL headers

https://github.com/stefan-niedermann/nextcloud-deck/issues/208

src/main/java/com/nextcloud/client/integration/AppCannotHandelNotificationException.java

@@ -1,3 +1,23 @@
+/*
+ * Nextcloud application
+ *
+ * @author Stefan Niedermann
+ * Copyright (C) 2020 Stefan Niedermann <info@niedermann.it>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
 package com.nextcloud.client.integration;
 
 public class AppCannotHandelNotificationException extends Exception {

src/main/java/com/nextcloud/client/integration/AppNotInstalledException.java

@@ -1,3 +1,23 @@
+/*
+ * Nextcloud application
+ *
+ * @author Stefan Niedermann
+ * Copyright (C) 2020 Stefan Niedermann <info@niedermann.it>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
 package com.nextcloud.client.integration;
 
 public class AppNotInstalledException extends Exception {

src/main/java/com/nextcloud/client/integration/deck/DeckNotificationHandler.java

@@ -1,3 +1,23 @@
+/*
+ * Nextcloud application
+ *
+ * @author Stefan Niedermann
+ * Copyright (C) 2020 Stefan Niedermann <info@niedermann.it>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
 package com.nextcloud.client.integration.deck;
 
 import android.content.Context;