Allow Expressions in Appearance Configuration

[dpfaffenbauer]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	yes
RFC?	yes

Having a configuration like this to have only certain bricks for certain "configs" could be easier done using expression engine.

toolbox:
    areas_appearance:
        newsletter:
            allowed:
                - newsletter_image
                - newsletter_columns
                - newsletter_text
                - newsletter_blub
       newsletter:
            allowed:
                - b2b_blub
                - b2b_blab
                - b2b_foo
                - b2b_bar

like this:

toolbox:
    areas_appearance:
        newsletter:
            allowed:
                - @=strpos(areaBrickId, 'newsletter_') == 0
        b2b:
            allowed:
                - @=strpos(areaBrickId, 'b2b_') == 0

Allowed values per configuration could actually be calculated within the container building time. If you like that, I could create a PR. This could also affect groups of bricks.