pty: Add a spwan flag to not inherit the parent environment

In gnome-terminal we don't want the gnome-terminal-server's environment
to pollute the child environment, so add the VTE_SPAWN_NO_PARENT_ENVV flag
that when passed to vte_terminal_fork_command_full() makes it *not*
merge the parent's environment.
prompt: Return 0 instead of last exit status
(cherry picked from commit 4100d11e579108289f7e3c994865e77aa7c35422)
emulation: Allow unsetting the current-{directory,file}-uri

By passing "" (or an invalid, non-URI value), unset the property.
(cherry picked from commit 89f6741d8e0ce481b7ec188043e26488edb5a455)
widget: Remove vte_view_im_append_menuitems