enum DiaObjectFlags, DiaObject fixed some dox and typos
pdf-import: cache images during load

rather than re-parsing them over and over again. Also this spares quite some
memory if an image is referenced more than once, now there is only a single
pixbuf for e.g. background images.
svg-import: improve 2 point closed path handling and use style

The latter is still a compromise, but leads to better results
with render-test round-trip through cairo SVG export. The former
was throwing a g_warning and is now just closing the path.
Give DiaContext* to more save functions

Should introduce no functional change. For object functions the previous
filename parameter is replaced by DiaContext *ctx. The few functions
using the filename got ported to use dia_context_get_filename()
DiaContext* for data_add_*() and PropertyType_Save functions

Now save is on par with load again, i.e. both have a context available
to bundled error messages instead of single message_*() calls. Also
this allows other fancy stuff coming next;)
Bug 600571 - Connections spanning layers can not be serialized

 - read and write connections after all the objects are serialized to
   have the right connection targets
 - serialize "connectable" state of the layer
 - fix layer widget initialization to properly reflect the loaded state
C++ guards for group.h
bindings: reduce bit rot

 - adjust test.py for some function renaming
 - implement dia.message() and dia:group_create()
 - use DiaContext in dia::ObjectType::save()
 - disable dia::ObjectType::get_defaults()
 - fix typos
transparency: fix diasvg_import.py to read colors again

This is reverting this files changes by the big commit
Bug #591525 - Transparent objects by adding alpha to Color structure
b8994a3489256de6baaabaf09a7cf69f39390353, which actually had two issues:

 - transparency in SVG is not represented by quadlets like #rrggbbaa
 - even if it would: a mismatch of the regex should have led to the
   fallback interpretation but was producing an exception instead
Give DiaContext* to more save functions leftovers
svg: avoid duplicated IDs in examples, if ...

not intentional for testing. Result is less errors from SVG
validation at http://validator.w3.org/
Improve robustness of dia_context_get_filename() use

Not setting the filename on the context should not produce
crashes. So don't return NULL with dia_context_get_filename()
but a valid string. Should be more reliable than adding
NULL pointer to all the users.