[ios] Add track's text/HTML description field like for bookmark

Now when editing a bookmark, a user can type text or HTML description. That's not the case (yet) when editing a track.

Need to reuse the same text/HTML displaying/editing elements as for bookmarks in the Track Edit screen, and on the Track Place Page (when a track is selected). Description in such case should be rendered as for a bookmark, right below the cell with color/list/edit buttons.

All necessary API to retrieve track's description is already in the core and is used on Android.