Use tabular numbers for song duration

Essentially force monospace for digits if the font supports it. This allows
durations to stay aligned with each other and prevents seekbar changing
size each second.

data/ui/PlayerToolbar.ui

@@ -204,6 +204,9 @@
             <property name="halign">start</property>
             <property name="valign">center</property>
             <property name="label">0:00</property>
+            <attributes>
+              <attribute name="font-features" value="tnum=1"/>
+            </attributes>
           </object>
           <packing>
             <property name="position">0</property>
@@ -228,6 +231,9 @@
             <property name="halign">start</property>
             <property name="valign">center</property>
             <property name="label">0:00</property>
+            <attributes>
+              <attribute name="font-features" value="tnum=1"/>
+            </attributes>
           </object>
           <packing>
             <property name="position">2</property>

data/ui/SongWidget.ui

@@ -146,6 +146,9 @@
                 <property name="valign">center</property>
                 <property name="hexpand">True</property>
                 <property name="single_line_mode">True</property>
+                <attributes>
+                  <attribute name="font-features" value="tnum=1"/>
+                </attributes>
               </object>
             </child>
           </object>

gnomemusic/views/songsview.py

@@ -128,7 +128,9 @@ class SongsView(BaseView):
         column_album.props.expand = True
         self._view.append_column(column_album)
 
-        duration_renderer = Gtk.CellRendererText(xalign=1.0)
+        attrs = Pango.AttrList()
+        attrs.insert(Pango.AttrFontFeatures.new("tnum=1"))
+        duration_renderer = Gtk.CellRendererText(xalign=1.0, attributes=attrs)
         column_duration = Gtk.TreeViewColumn(
             "Duration", duration_renderer, text=5)
         self._view.append_column(column_duration)