Currently Loupe leaks expressions created in-code. This
is because expressions need to be unwatched in order to be
dropped. Since we'd need to hold a reference to the watch
ourselves and unwatch them in dispose, our code would
become more cumbersome if we were to try to handle this properly.
This commit moves the expressions on LpPropertiesView to the
UI file, as GTK handles cleaning up the expressions it creates,
without any extra consideration on the app side.
Propagating the height resulted in new window dimensions not
fitting to images.
Use ScrolledWindow instead of PreferencesPage to have
control over propagate-natural-height.
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.
