Inform of song change on play restart, add playlist auto sorting.

Fixes #511

src/settings/playlistsettingspage.cpp

@@ -64,6 +64,7 @@ void PlaylistSettingsPage::Load() {
   ui_->checkbox_greyout_songs_play->setChecked(s.value("greyout_songs_play", true).toBool());
   ui_->checkbox_select_track->setChecked(s.value("select_track", false).toBool());
   ui_->checkbox_playlist_clear->setChecked(s.value("playlist_clear", true).toBool());
+  ui_->checkbox_auto_sort->setChecked(s.value("auto_sort", false).toBool());
 
   Playlist::Path path = Playlist::Path(s.value(Playlist::kPathType, Playlist::Path_Automatic).toInt());
   switch (path) {
@@ -126,6 +127,7 @@ void PlaylistSettingsPage::Save() {
   s.setValue("editmetadatainline", ui_->checkbox_editmetadatainline->isChecked());
   s.setValue(Playlist::kWriteMetadata, ui_->checkbox_writemetadata->isChecked());
   s.setValue("delete_files", ui_->checkbox_delete_files->isChecked());
+  s.setValue("auto_sort", ui_->checkbox_auto_sort->isChecked());
   s.endGroup();
 
 }

src/settings/playlistsettingspage.ui

@@ -70,6 +70,13 @@
      </property>
     </widget>
    </item>
+   <item>
+    <widget class="QCheckBox" name="checkbox_auto_sort">
+     <property name="text">
+      <string>Automatically sort playlist when inserting songs</string>
+     </property>
+    </widget>
+   </item>
    <item>
     <widget class="QGroupBox" name="groupbox_paths">
      <property name="title">