GdkContentFormatsBuilder is currently not introspectable, as it does not
have a GType. We can turn it into a boxed type, but we need to implement
memory management for it.
The current gdk_content_formats_builder_free() function returns a newly
constructed value, so we cannot use it as a GBoxedFreeFunc; additionally
copying a GdkContentFormatsBuilder contents would make it a bit odd, as
you could get multiple identical GdkContentFormats out of the copies.
A simple approach is to model the GdkContentFormatsBuilder API to follow
the GBytes one: use reference counting for memory management, and have
a function to release a reference, return a GdkContentFormats, and reset
the GdkContentFormatsBuilder state.
For language bindings, we can provide a get_formats() function that
returns the GdkContentFormats instance and resets the builder instance,
leaving the reference count untouched.
For C convenience we can keep gdk_content_formats_builder_free(), and
make it a wrapper around gdk_content_formats_builder_get_formats(), with
the guarantee that it'll free the builder instance regardless of its
current reference count.
https://bugzilla.gnome.org/show_bug.cgi?id=793097https://blogs.gnome.org/otte/2018/02/03/builders/
We are using `path` unconditionally, but it can be conditionally filled.
To avoid inconsistent internal state, and a compiler warning, let's
assert that the variable is always set.
This is a GSList of GFile and we want it so we can operate with lists of
files and text/uri-list.
I chose GSList over GList because that's what the GtkFileChooser API
uses, too.
