shell: move non-ShellGlobal functions from shell-global to shell-util

shell: move non-ShellGlobal functions from shell-global to shell-util

shell-global had become a dumping ground for functions that didn't
have anywhere else to be. Make shell-util the dumping ground instead,
and have shell-global only have methods that involve the ShellGlobal
object.

https://bugzilla.gnome.org/show_bug.cgi?id=648755