added git submodule for Scinterm

this is actually referencing the branch `sciteco-dev` of the
Scinterm Git mirror at https://github.com/rhaberkorn/scinterm-mirror.git

Later when preparing a SciTECO release, separate branches per SciTECO
release will be created that "fix" the Scintilla/Scinterm versions
used by that particular SciTECO release.