ui: use correct name for missing icon

According to Icon Naming Specification[1], the name "gtk-missing-image" is not a standard icon, and may not available in all themes (e.g. missing from the new Adwaita icon theme). Use "image-missing" instead.

The patch comes from metacity.[2]

[1] http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html
[2] https://git.gnome.org/browse/metacity/commit/?id=aa16325e8b938ca6d471fdf508a7276e577f8212