...and update to latest Mozilla sources.

Originally, this was a non-external onlineupdate module (plus correspsonding
top-level include/onlineupdate/ directory) that apparently contained sources
originally copied from Mozilla and subsequently modified in-place (plus, mixed
in, presumably some sources that were not copied from Mozilla but were our own
inventions).  To clean up this mess, this has been turned into a proper
external/onlineupdate module with a tarball containing the pristine external
Mozilla sources.

The sources for the onlineupdate-c003be8b9727672e7d30972983b375f4c200233f.tar.xz
tarball are taken, somewhat arbitrarily, from a recent
<https://github.com/mozilla/gecko-dev/commit/c003be8b9727672e7d30972983b375f4c200233f>
("Bug 1867784 - Force reflow all kids in the last column balancing reflow.
r=layout-reviewers,dholbert") trunk state, by running
`external/onlineupdate/generate-sources.sh ~/github.com/mozilla/gecko-dev` on a
Fedora 39 machine.

The layout of the tarball still mostly follows the old onlineupdate/ layout,
even if that deviates heavily from the actual source layout at
<https://github.com/mozilla/gecko-dev/>.  (And some files, which apparently are
not needed, anyway, lacked sources, see the "Missing source for" in
external/onlineupdate/generate-sources.sh.  And win_dirent.h/.cpp has meanwhile
been superseded by updateutils_win.h/.cpp.)  Merely newly included source files
are laid out in the tarball according to the actual source layout.

Any LO-specific modifications are made via patch files (rather than modifying
the sources inline, as was done in the past): external/onlineupdate/lo.patch
contains whatever modifications are needed to adapt the functionality, while
external/onlineupdate/gtk3deprecated.patch fixes

> workdir/UnpackedTarball/onlineupdate/onlineupdate/source/update/updater/progressui_gtk.cpp:97:21: error: use of undeclared identifier 'gtk_vbox_new'; did you mean 'gtk_box_new'?
>    97 |   GtkWidget* vbox = gtk_vbox_new(TRUE, 6);
>       |                     ^~~~~~~~~~~~
>       |                     gtk_box_new

to not use the deprecated gtk_vbox_new, which is hidden because we include
-DGTK_DISABLE_DEPRECATED in our GTK3_CFLAGS as per our configure.ac.

On Windows, the definition of __BYTE_ORDER__ etc. is needed because
workdir/UnpackedTarball/onlineupdate/include/mozilla/ says "Our supported
compilers provide architecture-independent macros for this", but MSVC doesn't
actually, so define here what would implicitly be defined by GCC.  Similarly, on
Windows -U_WIN32_WINNT is needed to undo -D_WIN32_WINNT=0x0601 in
solenv/gbuild/platform/windows.mk, which would cause

> workdir\UnpackedTarball\onlineupdate\include\mozilla/WinHeaderOnlyUtils.h(537): error C2065: 'FILE_ID_INFO': undeclared identifier

etc., despite the #include <windws.h> there.

Curiously, the original gb_CustomTarget_CustomTarget,onlineupdate/generated from
onlineupdate/CustomTarget_generated.mk had to be renamed to
gb_CustomTarget_CustomTarget,external/onlineupdate/generated when the file was
moved to external/onlineupdate/CustomTarget_generated.mk, as otherwise a
top-level `make CustomTarget_onlineupdate/generated` would have failed with "No
rule to make target..."  Also, as there is no gb_CustomTarget_use_unpacked, its
effect has been poorly mimicked for now in
external/onlineupdate/CustomTarget_generated.mk.

Similarly, as there is no gb_WinResTarget_use_unpacked, its effect has been
poorly mimicked for now in external/onlineupdate/WinResTarget_updater.mk.

The original onlineupdate/workben/test_dialog.cxx, which is actually code
written by us, has been moved to external/onlineupdate/workben/test_dialog.cxx.

The original onlineupdate/qa/ sources (which were apparently not used during the
build) have been preserved for now as external/onlineupdate/qa/, for
documentation purposes.

The original onlineupdate/astyle.options (which was apparently not used during
the build) has been removed.

Change-Id: I5ea606202e7837269e7b128e45af2f0b8c277f9e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/160492
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>