Use GTK Composite Templates (close #62)

Use GTK Composite Templates for GUI elements to clean up and simplify
the code for widgets and all UI elements. This includes splitting the
large “gtk.glade” file into smaller .ui files and the large “widgets.py”
file into smaller .py files.

mcg/albumheaderbar.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+
+
+import gi
+gi.require_version('Gtk', '3.0')
+
+from gi.repository import Gtk, GObject
+
+
+
+
+@Gtk.Template(resource_path='/de/coderkun/mcg/ui/album-headerbar.ui')
+class AlbumHeaderbar(Gtk.HeaderBar):
+    __gtype_name__ = 'McgAlbumHeaderbar'
+    __gsignals__ = {
+        'close': (GObject.SIGNAL_RUN_FIRST, None, ())
+    }
+
+    # Widgets
+    standalone_title = Gtk.Template.Child()
+    standalone_artist = Gtk.Template.Child()
+
+
+    def __init__(self):
+        super().__init__()
+
+
+    @Gtk.Template.Callback()
+    def on_close_clicked(self, widget):
+        self.emit('close')
+
+
+    def set_album(self, album):
+        self.standalone_title.set_text(album.get_title())
+        self.standalone_artist.set_text(", ".join(album.get_albumartists()))