support ISO-date-format in plotlines/plotbands

[daemth]

relates to #23

[frankbaele]

The location of the input field is currently so generic its hard to predict if a date or normal value is needed. On top of that, the value field can be date or number depending on the current state of axis type.

So if we want to implement this we need to create plotline/plotbands specific components who can handle this use case. So i like to pospone this to 4.x since we are rethinking already how we build the ui.