[MlMt] Group Submailbox peer weeek

[Michael Nietzold]

i like to make a submailbox for each week in year

where:
- i create a submailbox
- i select a date

what i see:
- i can only select some parts of the Date

what i expected:
- it should be a better "custom selector" for Submailbox unique value 
selector (just now i can only select an message field but not the 
modifiers
- i see an manual section for the modifiers available in the  "Mailbox 
Name Format"
- take a format string to format the date like "yyyy-ww" to get it 
formatted