app: Fix build for OSX too.

This was broken in 7f23fbec34bd5790f0fab41a0975beed2786b8d8
This is similar to a25defa39e404f9e48863635ba2ac837559f573d
Document other layer_group-relevant procedures in gimp_layer_group_new.
Fix typo in header guard

Causes a warning when building with clang
Fix missing HARFBUZZ_LIBS in app/config
Fix stray semicolon causing early return

_wrap_gimp_color_scale_new will always fail because "return -1"
falls outside of the if statement.
There only *might* be an error string.
app, libgimp: commit files generated by PDB docs change
app: make sure image windows end up on the intended monitor

In gimp_image_window_new(), place the window on the right
monitor manually if we are not in single-window-mode.

In gimp_image_window_switch_page(), make sure we don't use an
unrealized new window's monitor for updating the session info, because
the monitor of an unrealized window is always where the pointer is.
devel-docs: more TODO items for GIMP3-API-Changes.txt
po-script-fu: update all script-fu's po with recent xgettext.

Current dev version of gettext has a fix for GIMP's script-fu syntax.
See Bug 720052.
Bug 720052 - Make GIMP recommend a recent gettext.

Script-fu gettext strings were mostly not extracted by xgettext. This
will be fixed in upcoming gettext 0.19. Add a test to warn packagers
using older gettext.
app: Update coding style (minor)
build: Lower HarfBuzz requirement to 0.9.26
app: update GimpDisplayShell's monitor resolution when changing monitors

so the image size adapts when dot-for-dot is off.
app: s/g_warning/g_printerr/ when GDK returns a bogus resolution

This is not a programming error, so no g_warning() must be used.
app: remove includes in gimpsessioninfo-dock.c and -book.c
build: Lowering HarfBuzz version further to 0.9.19 to support older distros
build/win: Upgrade fontconfig to 2.11.1
app: connect the warp tool's effect size to the standard tool size action
app: fix gimp_widget_get_monitor()

Do the right thing for widgets that have a window when the widget is
realized, and simply call gimp_get_monitor_at_pointer() if it's not
realized.