Add title and 'show more' to the List tile

Most tiles have a title, but not the List tile.  The List tile has a lot in common with the Collection tile, but it does not have a 'read more...' link.  I propose to make these tiles more alike, making it easier to have a consistent look-and-feel.

So this pull request enables editing the List tile on the Compose tab.  You can give it a title, optionally select an item to point to as 'read more...' link, and give the text to use as 'read more...' link.

One use case is: you want to list a few hand-picked pages with important information and then link to a folder containing more pages.

One technical choice deserves pointing out. We could have used the standard related items widget, but that uses a popup and the edit form of a tile is itself already a popup.  So that seemed too confusing.  Also, by default the second popup would show behind the first popup, though that could probably be fixed with some css.  So we reused the search part of the contentchooser of collective.cover. It then looks roughly the same as the edit form of a collection portlet.