allow more complex prerequisiteTags for fields

[tyrasd]

Prerequisite tags might be more complex than a simple key=*, 'key=value, 'key!=value or !key filter. Examples might be !key or key=no or key1=* and key2=* or key=value1 or key=value2. It would be useful if such situation could be expressed in the tagging schema.

[tordans]

Another example is the parking:SIDE:orientation=* which should only be visible when parking:SIDE=* is…

• present (not null)
• not no or separate

[tordans]

We also need the capability to support multiple prerequisiteTags, so the current object should become an array. This will open the question, if the array uses and AND or OR logic. I think only going for AND would be fine. But we would have to test this out with more cases. Update: #153 needs an OR.

Update: See #153 (comment) for a example that uses the array-format with an OR.