reuse of open documents generalized. Refs: #34.
Changes as proposed in CPC 1.7 rc5.
The core of the changes is the new procedure Windows.GetBySpec,
which centralizes the reuse behavior based on file name, location,
converter and document flags. Reuse happens only for documents
with the same flags, e.g. an aux window is only reused for
another aux window. Thereby it is possible to edit and browse
a document independently as it was before.

Signed-off-by: Josef Templ <josef.templ@gmail.com>