List in widget

This comes in place of my previous pull request (https://github.com/openintents/shoppinglist/pull/9), which can be closed (sorry, I couldn't figure out how to do that).

Compared to the previous pull request I removed the 'clean list' button from the widget, and also refactored it, so that I now use Build.VERSION instead of the bools.xml.  I wrote in that previous pull-request that the android 2 version seemed broken (the next/prev buttons didn't work), but I think that problem was actually caused by something I did, for in this version they do work.

What is changed in this branch is:
* Check an item in the widget doesn't make it disappear immediately, but grays it out (this gives better feedback as to what happenend, and gives the possibility of an undo)
* For android honeycomb or higher, the widget now uses a ListView.  This looks better (in my opinion), especially if multi-line items are used, and makes the next/prev buttons unnecessary.
* Some small changes to the layout of the widget: the buttons are aligned to the top now, the default size is smaller, so that it can also be displayed on (very) small phones.