allow to mark species using stored selection in merge tool

[westram]

Goal:

Provide access to species selections in merge tool.

Implementation:

• inspect code of species selection windows. Make sure it is multi-usable, i.e.
  • independent from globals (database has to passed to window creator),
  • multiple separate windows may get instantiated (pass ID-prefix and optional title-prefix).
    Make sure macro-IDs do not change for "normal" species selection window!
• allow to open species selection windows from 'Transfer species' window.
• test changes interact well with 'species selection transfer'