Add license headers to UI files

With links back to base projects.

data/ui/add_contact.ui

 <?xml version="1.0" encoding="UTF-8"?>
+<!--
+  Copyright (c) 2019 Nico Alt
+  SPDX-License-Identifier: AGPL-3.0-only
+  License-Filename: LICENSE.md
+-->
 <interface>
   <requires lib="gtk+" version="3.20"/>
   <object class="GtkGrid" id="add_contact">

data/ui/chat.ui

 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Generated with glade 3.22.1 -->
+<!--
+  Copyright (c) 2019 Nico Alt
+  SPDX-License-Identifier: AGPL-3.0-only
+  License-Filename: LICENSE.md
+-->
 <interface>
   <requires lib="gtk+" version="3.20"/>
   <object class="GtkGrid" id="chat">

data/ui/login.ui

 <?xml version="1.0" encoding="UTF-8"?>
+<!--
+  Copyright (c) 2019 Nico Alt
+  SPDX-License-Identifier: AGPL-3.0-only
+  License-Filename: LICENSE.md
+
+  Based on parts of GNOME Fractal
+  https://gitlab.gnome.org/GNOME/fractal/blob/4.2.2/fractal-gtk/res/ui/login_flow.ui
+-->
 <interface>
   <requires lib="gtk+" version="3.22"/>
   <object class="GtkStack" id="login_flow_stack">

data/ui/main.ui

 <?xml version="1.0" encoding="UTF-8"?>
+<!--
+  Copyright (c) 2019 Nico Alt
+  SPDX-License-Identifier: AGPL-3.0-only
+  License-Filename: LICENSE.md
+-->
 <interface>
   <requires lib="gtk+" version="3.20"/>
   <object class="GtkListBox" id="contacts_list">

data/ui/registration.ui

 <?xml version="1.0" encoding="UTF-8"?>
+<!--
+  Copyright (c) 2019 Nico Alt
+  SPDX-License-Identifier: AGPL-3.0-only
+  License-Filename: LICENSE.md
+
+  Based on parts of GNOME Fractal
+  https://gitlab.gnome.org/GNOME/fractal/blob/4.2.2/fractal-gtk/res/ui/login_flow.ui
+-->
 <interface>
   <requires lib="gtk+" version="3.22"/>
   <object class="GtkStack" id="registration_flow_stack">

data/ui/toolbar_start.ui

 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Generated with glade 3.22.1 -->
-<!-- Based on https://gitlab.gnome.org/World/lollypop/blob/1.0.12/data/ToolbarPlayback.ui -->
+<!--
+  Copyright (c) 2019 Nico Alt
+  SPDX-License-Identifier: AGPL-3.0-only
+  License-Filename: LICENSE.md
+
+  Based on parts of GNOME Lollypop
+  https://gitlab.gnome.org/World/lollypop/blob/1.0.12/data/ToolbarPlayback.ui
+-->
 <interface>
   <requires lib="gtk+" version="3.10"/>
   <object class="GtkImage" id="back_image">