A bunch of small additions that have been on the todo list forever

Added way to force run unit tests by right clicking on assignment in gradebook.
Changed submit date in file browser to use new date class
Added right click menu to show/hide modifications in the filebrowser
    Stores that value in the config
Demo mode can no longer delete files
Gradebook now shows date graded in a tooltip (along with unit tested)
Filebrowser sort now sorts properly for assignment numbers and dates.
Code tabs can now be closed with right click on them.
When grade column is hidden, grades are also hidden in the grader view now