main: Set the dark theme in two lines.
application: Catch errors.
application-window: Make window children read-only properties. It's a
little uglier, but it's the right thing to do.

Also, use [ and ] as shortcuts for left and right rotate; they
actually work.
application: Set some variables only after it's clear they will be
used.
everywhere: Consistently use gqpe instead of quick-photo-editor.
README: Yee Kee Pee Yay.
README: Or not.