[cleanup] Make DiaObject::enclosing_box an optional pointer

Only three objects are currently using the extra storage, but it did
occupy four double values (32 bytes) in every single DiaObject.
Now it's only one pointer and some indirection with the three objects,
but faster dia_object_get_enclosing_box() for all objects and less
memory usage for most.

test-sizeof (before)
DiaObject: 136
Connection: 264
Element: 432
UMLClass: 992
Handle: 32
ConnectionPoint: 32

test-sizeof (after)
DiaObject: 104
Connection: 232
Element: 400
UMLClass: 960
Handle: 32
path: handle special cases of combinations w/o crossing

 - path_combine(): implement the respective !crossing branch
 - create_standard_path_from_list(): may return empty now
 - _combine_to_path_callback(): an empty result is like a delete
   Also fix transaction point position.
dox: update developer documentation for SVG renderers

Fix the issue marked and listed as Doxygen bug by, it was just the
wrong name for \memberof.
Fix typos and document potential side-effect of DiaObject::move()

The recent crash with "UML - Component Feature" could have been avoided by
using the trik to make an object update it's data. With the test this is
not wanted. Ideally we would not need that trick anywhere.
gdk-renderer: don't try to stipple completely transparent

Wit very big drawings this was failing deep in gdkdrawable-win32.c(generic_draw)
while creating huge mask and tile bitmaps for not doing anything with them.
Now GDK drawing is just avoided with alpha==0.0.
svg: optimize SvgRenderer::draw_object() with matrix

by inlining the group drwaing we can create a single transform for the
whole group rather than transformation groups for every object in it.
Improve clipboard handling for OS X

 - Use gtk_clipboard_wait_for_targets() and select based on that to paste
   either SVG or fallback to image
 - Add TIFF and PNG to the supported formats
 - Use the global clipboard as with win32. Only X11 uses the "selection"
   keyboard
 - try to treat UTF8_STRING as SVG (works with iDraw/Copy/SVG)
Bug 736473 - pdf-import.cpp build issue on 64-bit systems

Not using %p because that would give a C run-time dependent prefix.