`parse_svg_file_future()` is called in a cancellable async
block. This means that after it's called it can be cancelled,
which drops the receiver. When we fetch the metadata, we try
sending the result to the dropped receiver.
This is expected behavior in this case - the receiver
being gone at the time of sending the metadata is perfectly
valid. However, previously we would cause the thread to panic.
This didn't break anything but made for scary log message.
Since this is expected behavior we can use `_` to tell the
compiler that we don't care about the outcome of sending.
This is valid in this instance because we will only get an
error if the receiver is gone.
Implement a properties sidebar. In order to prevent
the UI from freezing, we use futures to load and cancel
loading file information. We also make use of
`gdk_pixbuf` and `svg_metadata` in order to get the file's
metadata before the file fully loads, meaning there's
little to no delay between changing files and seeing
the metadata for large images.
The properties widget uses expressions and
properties to avoid glue code for setting each label,
allowing us to have a clean separation between UI
and data.
