This was also needed in other parts of the UI and is easier to implement
in the target class anyway.
Note that DEAD is now properly restricted to air defense targets.
Also added error boxes to the UI for when planning fails on an invalid
target.
`FrontLine` is tightly coupled with `ConflictTheater`.
Moved into the same module to prevent circular imports.
Moved `ConflictTheater.frontline_data` from class var
to instance var to allow save games to have different
versions of frontlines.
