Display images in a list view

The main activity did only display a text view.

Now it displays a list view with all the pictures found in the media
store, in the external content directory.

Changes:

- In main layout, add a list view with the standard android id
- Create a layout for the list items with two text labels
- In activity:
    * Extend list activity class
    * Use a list adapter for displaying the pictures. It uses
    a cursor on the media store to scan the pictures, and the list
    items' layout to display them.
    * When clicking on an item, show a toast with its path