Tidy up unnecessary Linq usages

This PR removes ```using System.Linq``` statements and replaces Linq with plain loops or different calls in a few places.

As part of removing Linq from TouchCollection I lined it up with WP7 behaviour (doesn't allow null in the constructor) and gave it its own Enumerator Implementation to match XNA also.

Have split this in to 3 commits so they can be reviewed individually as necessary.
ec6c4afe821bf2b798965a07786051ff4782a200 is just removing using statements.
65647cb31547b5410740c8e354a9886b3084566d is replacing Linq usages.
7899705d680f2bad8de6243d48c606f2a13ac706 is the TouchCollection touch-up.

TouchCollection ref: http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.input.touch.touchcollection.getenumerator.aspx
Refs #2717