playback-history-qt: Set a menu icon

There is no fitting icon available in all themes but "view-history"
looks well at least on KDE with the Breeze or Oxygen icon theme.

src/qtui/menus.cc

@@ -166,7 +166,7 @@ QMenuBar * qtui_build_menubar(QWidget * parent)
             {N_("Song _Info ..."), "dialog-information", "Ctrl+I"},
             audqt::infowin_show_current),
         audqt::MenuCommand(
-            {N_("Playback Histor_y ..."), nullptr, "Ctrl+H"},
+            {N_("Playback Histor_y ..."), "view-history", "Ctrl+H"},
             show_playback_history),
         audqt::MenuSep(),
         audqt::MenuCommand({N_("Set Repeat Point _A"), nullptr, "Ctrl+1"},

src/skins-qt/menus.cc

@@ -90,7 +90,7 @@ static const audqt::MenuItem main_items[] = {
 
 static const audqt::MenuItem playback_items[] = {
     audqt::MenuCommand ({N_("Song Info ..."), "dialog-information", "I"}, audqt::infowin_show_current),
-    audqt::MenuCommand ({N_("Playback History ..."), nullptr, "H"}, action_playback_history),
+    audqt::MenuCommand ({N_("Playback History ..."), "view-history", "H"}, action_playback_history),
     audqt::MenuSep (),
     audqt::MenuToggle ({N_("Repeat"), "media-playlist-repeat", "R"}, {nullptr, "repeat", "set repeat"}),
     audqt::MenuToggle ({N_("Shuffle"), "media-playlist-shuffle", "S"}, {nullptr, "shuffle", "set shuffle"}),