Store options with GSettings

Add GSettings support to give some window options persistence.
Remove sigc decltype macro

It isn't actually needed currently, and I don't know that it does
anything from where it is.
Small cleanup for ImageView

Tweak the code to look a little nicer.
Add gschemas.compiled to CLEANFILES

'make clean' now removes this file.
Update zoom when the window is resized

In zoom-to-fit mode, this keeps the image size synced with the window
size.
Remember window's maximized state

Save the maximized state to a setting and restore it on startup.
Add border between list and image

Small visual change to better separate the image from the file list.
Encapsulate ImageView's Gtk::Image

Store the image widget as an automatic variable in the ImageView class,
so that the UI file doesn't need to know about it, and so ImageView
doesn't need to hardcode a specific widget ID for Gtk::Builder. This is
more flexible.
Add .desktop file

Also change the icon to image-x-generic since it has a larger size than
emblem-photos.
Update screenshot