When pasting text in the speed reader, query only the main clipboard

This mostly reverts the previous related commit as it turned out that
it would most often paste the selected text from the search field
which we don't want.

lib/speedreader.c

@@ -548,14 +548,9 @@ static void sr_clear_clicked_cb(GtkButton *button, GtkTextBuffer *buffer)
 
 static void sr_paste_clicked_cb(GtkButton *button, GtkTextBuffer *buffer)
 {
-	gchar *text;
-	gtk_text_buffer_set_text(buffer, "", 0);
-	text = dict_get_clipboard_contents();
-	if (text != NULL)
-	{
-		gtk_text_buffer_set_text(buffer, text, -1);
-		g_free(text);
-	}
+	GtkClipboard *clipboard = gtk_clipboard_get(GDK_SELECTION_CLIPBOARD);
+ 	gtk_text_buffer_set_text(buffer, "", 0);
+	gtk_text_buffer_paste_clipboard(buffer, clipboard, NULL, TRUE);
 }