Add WatchList macro

Idea: factor out a macro that displays the current users watchlist
and allow users to add this macro as a gadget on the dashboard.

Recently I have figured out that users tend to see the "watchlist" feature as "bookmarks" / "favourites", i.e. they like to watch pages not to get information about changes but to find them faster.
Of course the watchlist section in the user profile can serve that purpose, but at least some users find this not very intuitive.

So here is what I would like to do:

• extract the relevant code from XWiki.XWikiUserWatchListSheet displaying the watchlist into a separate maco on a page e.g. XWiki.WatchListMacro
• use this watchlist macro in the above sheet to avoid code duplication.
• take care the wachlist macro can handle anonymous users (if used on a publically visible dashboard)
• (optionally) for local users (i.e. those only present in a subwiki) scrap the "wiki" column in the table, as it does not add any valuable information for them