glib: Fix missing destructor call

PopplerAttachmentPrivate has a Object member which
was never destructed, only set to an empty Object()
on dispose. While there is no memory leak (currently!),
this is still not correct.

Fix this by making  PopplerAttachmentPrivate a C++ class,
constructed in place of the gobject instance private in
init(), and call the destructor explicitly in finalize().
4 jobs for mr/glib-destructor in 35 minutes and 14 seconds (queued for 1 minute and 32 seconds)
Status Job ID Name Coverage
  Build
passed #39616
build

00:09:00

passed #39617
build_clang_libcpp

00:09:09

passed #39618
build_gcc_49

00:12:03

passed #39619
build_mingw64_fedora

00:05:00