Implement Properties

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.

src/widgets/mod.rs

@@ -20,7 +20,9 @@
 mod image;
 mod image_page;
 mod image_view;
+mod properties_view;
 
 pub use image::LpImage;
 pub use image_page::LpImagePage;
 pub use image_view::LpImageView;
+pub use properties_view::LpPropertiesView;