Previously the unchanged contents check tested both `key` and `value`
at the same time, so the following case was not accepted.
`ENV_VAR=ENV_VAR`
No matter how unlikely it is for the key to be equal to the value, there
is no reason to restrict it.
For the `key` ("0") column, protecting against existing keys also handles
this case. For the `value` ("1") column, a check has been added.
Using a ChainMap and this custom model, we can display global environ
variables in the per-game settings, allowin better overview and
simpler override.
