Refactor handling of valid account names

• Move our system_names and system_groups out to a util header.

• Qt UI: Validate that the user's chosen account name is not contained in either system_names nor system_groups.

Closes: #388 (closed) Acked-by: @Sheila

The UI will look like this (except more KWin and less Aqua):

changed milestone to %0.9.9

added Qt UI Script Processor labels

requested review from @z

assigned to @awilfox

changed the description

merged