Customizable backend content listings.

Finally the first incarnation of customizable backend content listings is ready and open for discussion.

I think I found a way an unintrusive way to add configurable backend content listings. Quite flexible but still easy to use.

Simply throw a file named <contenttypename>.twig into config/view/listing/ and it gets used for display. You can define blocks for id, meta, action and content columns. If a block is not defined the default definition is automatically used. So you will often simlpy end up defining a content block and the header row and don't have to care about the rest. And you have to define the column count if it differs from the default as it is needed for the group column.

A (nonsense) example template is added to see it working out of the box. Simply remove the .dist extension.

I also fixed a little bug, where the header row isn't displayed in non grouped listings.

TODO:
- possibility to add some user CSS to the backend to make it possible to style your added columns
- have a closer look at grouping/sorting as I think hat there's something wrong with