Bug 733824 - PDF export fails without warning if open in other application.

This happens on the Windows platforms in particular where applications
can lock files, preventing GIMP to obtain a file descriptor for writing.
app: filename -> GFile in parse_iso_codes()
app: more GFile stuff in gimp_get_temp_file()
plug-ins: use standard error on failed file descriptor in file-pdf-save.

The standard message used all over our code is:
"Could not open '%s' for writing: %s".
app: add --verbose output when parsing and writing the action history
build/win: Upgrade libpng to 1.6.12
build/win: Disable lto in cairo

It causes issues with older binutils versions. See:
https://bugs.freedesktop.org/show_bug.cgi?id=77060
build/win: Mark poppler as optional

Compiling it tends to fail on newer mingw releases.
app: port GimpAppleEventHandler to GFile too, it made opening files crash
build/win: Reset tarball location on newer jhbuild versions.
build/win: Add support for building 64-bit libs in jhbuild.
build/win: Disable libwmf for now.
app: s/uri/file/ in a comment in file-open.c
app: typo s/fomat/format/
app: port GimpDataFactory file loading to GFileEnumerator

Also slightly change the semantics of gimp_data_set_folder_tags()
to be less complicated.
app: move gimp_file_compare() to gimp-utils.[ch]

It's a GCompareFunc to be used for finding GFiles in lists.
app: don't check for a writable dir for each loaded GimpData

Instead, do the check once for each element of the data path, and pass
the result to the load functions.
app: use g_str_has_suffix() in file_utils_file_get_ext()

g_strrstr() is wrong for checking for suffixes.
app: add gimp_file_has_extension() and use it in GimpDataFactory
libgimpmodule: always build the module debug code, just don't call it

It was already rotting and had to be fixed.