str.replace requires 0.58

webkitgtk is not required to build libsushi and is only used by the HTML
viewer, so allow sushi to work even in its absence.

We pass unescaped text to GtkLabel.set_markup.  For type, size,
and date formatted, this is probably fine, but it causes problems
with filenames.  If, for example, the filename has an "&" in it,
the filename field will be unavailable.

There's a bug where the call to InstallProvideFiles can end up in an
infinite loop because the callback is called prematurely which ends
up calling load_libreoffice.

Modify2 has been around for many years, removes an unnecessary call
to get xid (which is incorrectly called on Wayland).  It appears that
Modify2 doesn't end up in the infinite loop.

See: https://gitlab.gnome.org/GNOME/gnome-software/-/commit/88e90c7ebe2e6ebfd7abd77a40baf479994f51ff

Fixes: https://gitlab.gnome.org/GNOME/sushi/-/issues/60

We have to add the tick callback to the overlay widget instead of the main
widget, otherwise the position bar is not updated in fullscreen mode when the
gtksink is used. This happens because we move the overlay between the main and
fullscreen windows, and we did't get the events when the overlay is outside of
the main widget.