plug-ins: include libgimp/gimp.h and gimpui.h in openexr-wrapper.cc

The file does not need the includes, but including libgimp from a C++
file makes sure the build fails if something forbidden gets added to
any public libgimp header, such as a struct member named "private".
devel-docs: add more stuff to GIMP3-API-Changes.txt
Updated Greek translation
plug-ins: pass "RAW" to gimp_export_image() in file-raw-data.c

So the export logic GUI becomes debuggable.