Add mimetype support for system testcases to qml editor

System test-cases are (assumed to be) written in javascript when using
an .qtt filename extension. These modificatons ensure that a js editor
is used whenever an attempt is made to open a file that ends with .qtt.

Also add support for recognising testcases to QmlOutlineModel.

Change-Id: Ibcb68126e5123e8069344cf0c05aa2396b967a12
Reviewed-on: http://codereview.qt.nokia.com/2259


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>
Reviewed-by: Bill King <bill.king@nokia.com>