PIDE-phase-2a: xml_of_tac: different substitutions converted to same XML

Note: presentation and handling of substitutions in the frontend
is still unclear.

PIDE-phase-2a: corrected XML conversion for Tactics

Note: handling of substitutions on the frontend (input!)
is still unclear.

PIDE-phase-2a: corrected XML conversion for intree of appendFormula

Note: there is still a hack caused by insufficient Java-knowledge
cf. https://intra.ist.tugraz.at/hg/isac/file/033c8d5e8ea9/isac-java/src/java/isac/util/formulae/CalcFormula.java#l23

PIDE-phase-2a: xml_to_* for operation_setup of all Math_Engine

cf. https://github.com/wneuper/libisabelle/commit/4ab04dd1a8bfe9d2d4d174c216d637b233ed42dc
phases described at https://intra.ist.tugraz.at/hg/isac/rev/232e68e6cad4.

Note: Communication isac-java <--> Isabelle/Isac by "isabelle tty"
was only partially prepared for asynchronous communication,
see https://intra.ist.tugraz.at/hg/isa/file/68ca125a58cb/src/Tools/isac/xmlsrc/interface-xml.sml#l5

In PIDE-phase-2a this partial state is left as is
wrt. funs "*OK2xml", "*ERROR2xml", etc;
focus is the translation to XML.tree.

PIDE: auxiliary funs for operation_setup apply_tac

cf. https://github.com/wneuper/libisabelle/commit/31decc2ebddacbcd06a2bbc21ddbab494751a377

PIDE: auxiliary funs for operation_setup append_form

cf. https://github.com/wneuper/libisabelle/commit/8db4fe8eac32603184e70b9f225e570270e35f0a

PIDE: reorder structure Math_Engine for implementation of interface in libisabelle

cf. https://github.com/wneuper/libisabelle/commit/ae30ad91eba871fcad8f69e4151d4e783e20c3ff
cf. https://intra.ist.tugraz.at/hg/isac/rev/419b45750882

PIDE: improved error-msg during embedding into Java-side

Note: improvement of exception handling is out of scope presently.

PIDE: corrected XML generation

now according to
https://github.com/wneuper/libisabelle/blob/master/doc/test--isac-Java--isac-kernel.txt

PIDE: added missing code in interface-xml