Use per-task slot in ImageWorker

Instead of having a class signal that you connect to once, require a
slot for every call to load(). This allows variables unique to each task
to be bound to the slot, reducing coupling between ImageWorker and its
callers. (Namely, the dependency on Gtk::TreeIter in image_worker.h is
removed.)
Support file opening in the .desktop file

Add %u to the exec line to support opening a URL, and add some common
image MIME types.
Decrease minimum zoom to 10%
Remove intermediary zoom_[in|out|normal]() funcs

Instead, connect ImageView's functions directly to the window actions.
Update README

Re-word some stuff.