Add "working hour windows" for employees

[DBJRdev]

An outer working hour limit (e.g. mornings from 6:00 till evening 20:00) should be added on a per installation basis, separately for each day of the week and also once for all holidays. The limit is not supposed to be strict on worklogs, but a indicator should be visible in the monthly worklogs.

Because such days may change over the years, a start date and optional end date of such framing hours need to be added.

Use of it: working before or after these hours need extra agreements and extra compensation by labour regulations. First step is to actually be able to identify such hours.