Fix compilation with ICU 68

nip2 depends on the ICU library via libxml2. ICU released version
68 on 2010-10-28, and switched to using the C99 bool type in its
public headers. nip2 was using "bool" as the name for a gboolean
in the ParseConst struct. This commit renames this to "boolean".

See #84

Thanks Matt

icontainer and watch were declaring some signals as taking objevct
arguments, but using a simple pointer marshal function. This is harmless
on Debian-dervived distros, but redhat seem to build glib with
run-time time checks which spot this and NULL out pointers, causing a
crash.

See #75

would display -ve values for pixels > 2**31

see #74

and some small cleanups

fix build with libvips8.7 changes ... you unow need vips7compat.h to
build with the vips7 API

add the extra `vips7compat.h` we will need with libvips 8.7

"header" just displays enum nick

Some PNG files written with some versions of libpng 1.6 fail to read on
current libpng ... we had one and it broke `make check`. Fix the file by
copying to an old machine, saving as tiff, copying back, and resaving as
png again.

See #72

we don't need flex lib since we define our own input

we could produce spurious compat warnings before

see hist_find_indexed

improve RAD handling
bump version
see #65

just the .desktop file to fix