Make both gtk_grid_view_new and gtk_grid_view_new_with_factory
take a model as first argument, and make all arguments
allow-none and transfer full.
Update all callers.
In 99.9% of all cases, these are just NULL, NULL.
So just do away with these arguments, people can
use the setters for the rare cases where they want
the scrolled window to use a different adjustment.
According to [1], '_timezone' is already used for a global variable in the
time.h system header that is supplied by Microsoft, so using that for our
variable name when we are including time.h either directly or indirectly
will cause trouble.
This renames such variables to '_tz' to avoid that
[1]: https://docs.microsoft.com/en-us/cpp/c-runtime-library/daylight-dstbias-timezone-and-tzname
