[pitivi] mediafilespreviewer: Stop using deprecated set_stock_id

[Alexandru Băluț <alexbalut src gnome org>]

commit d1d9b42facbd45dcfbf07865cc27805181ccf2e2
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Tue Mar 26 01:57:21 2019 +0100

    mediafilespreviewer: Stop using deprecated set_stock_id

 pitivi/mediafilespreviewer.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/pitivi/mediafilespreviewer.py b/pitivi/mediafilespreviewer.py
index 090f6000..6c05e94d 100644
--- a/pitivi/mediafilespreviewer.py
+++ b/pitivi/mediafilespreviewer.py
@@ -313,7 +313,7 @@ class PreviewWidget(Gtk.Grid, Loggable):
             self.player.simple_seek(0)
         self.player.setState(Gst.State.PLAYING)
         self.is_playing = True
-        self.play_button.set_stock_id(Gtk.STOCK_MEDIA_PAUSE)
+        self.play_button.set_icon_name("media-playback-pause")
         GLib.timeout_add(250, self._update_position)
         self.debug("Preview started")
 
@@ -321,7 +321,7 @@ class PreviewWidget(Gtk.Grid, Loggable):
         if state is not None:
             self.player.setState(state)
         self.is_playing = False
-        self.play_button.set_stock_id(Gtk.STOCK_MEDIA_PLAY)
+        self.play_button.set_icon_name("media-playback-start")
         self.log("Preview paused")
 
     def togglePlayback(self):