Clean up grid selector at login

This is pretty much done, except that it crashes when the previous session's grid selection is not recognized in the list.

This is a rather big problem, since that applies to the first startup after using a previous version of Imprudence. And, since the user can never log in (and thus the grid selection never changes to a valid value), it crashes at every startup thereafter unless the user edits some settings.

The proper behavior would be for the login screen to add an "Other" option to the login page and select that when the grid is not recognized.

Ideally, the old selection of "Agni" would automatically be changed to "SL Main Grid", too.

I've reverted to an older, non-crashy revision and am now calling it done.

The code changes I made that were triggering the crash were just to prevent the previous session's grid selection from being listed twice in the grid selector. We can live with that little quirk for a while.