sanitize lupdate's behavior regarding SUBDIRS projects

previously, the semantics of nesting projects were pretty bizarre:
the list of TS files grew ever as the subdirs were visited. the
source files otoh were cleared for every project. this meant that
some TS files were updated over and over again, each time with different
data.
this is obviously total nonsense, so there is no compatibilty to keep.
so here come the new semantics:
- each project is scanned separately. it has separate include paths and
  may override the inherited CODECFORSRC.
- the messages from all sub-projects are merged
- if a sub-project has a TRANSLATIONS entry, it is "detached" from its
  parent project, thus starting an own merged translator. it is also
  possible to name an empty set of TS files to simply exclude the
  sub-project.
- CODECFORTR can be specified in each project with TRANSLATIONS
- if TS files are specified on the command line, they override
  TRANSLATIONS from the top level project and stop processing of
  detached projects alltogether. if multiple top-level projects are
  specified, they are all merged.

this is somewhat slower, as now includes are re-scanned per project,
while previously all sources were simply accumulated and scanned as one
project. this can be fixed retroactively if needed.
(cherry picked from commit b62b87d6ae3ced4466c1a1b085b51cec757e24f5)

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