Export file options should warn when removing nickname placeholder

[btbest]

The file export settings dialog allows editing the export path. The user can potentially remove the {nickname} placeholder, in which case the export path for all lanes will be identical. This means the exports will overwrite the one file repeatedly, and the user ends up with only the last lane's export.

The dialog should show some sort of warning when the user tries to enter a path without the nickname placeholder, if there is more than one lane.

Not sure if there are other placeholders that would discriminate the lanes; if so, their presence should be checked before showing the warning.