Handle KP keys in copy/paste keybindings

We currently handle KP variants of some keys, but not consistenly.

https://bugzilla.gnome.org/show_bug.cgi?id=364566
docs: Explain 'icon theme context' better in gtkicontheme.c

Add links from gtk_icon_theme_list_contexts() to
gtk_icon_theme_list_icons(), and from there to the Icon Theme
Specification and the Icon Naming Specification.

https://bugzilla.gnome.org/show_bug.cgi?id=461249