Allow context-specific extension of ACI language

[westram]

Motivation:

• specific contexts where ACIs are used, should allow access to specific information, e.g.
  • size of or ali-filter used during generation (in selection lists; see #758)
  • values that are retrieved during group-search (e.g. group-size)
  • index of hit in result-list (various search tools) ⇒ will allow to generate differing (non-duplicate) names (e.g. for scripted group-rename)

Implementation:

• values should be provided via ACI-plugins. A plugin should extend the ACI-language using an abstract feature definition, e.g. define the commands "treesize" or "groupsize" for the above examples.
• plugin has to define
  • function name
  • callback (gets input-streams + parameters; returns output-streams)

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required by: #758
related: #763

[westram]

by [16374]