tab: use gedit_tab_get_view() and gedit_tab_get_document()

It's shorter, and if one day the code of those functions change, it must
be changed at only one place.
tab: no need to triple-check the auto save feature

Keep the auto save independent of the other features.
gedit_tab_set_state() does the right thing.
tab: set auto_save_timeout to 0 before setting the state

gedit_tab_set_state() will remove the timeout, but the timeout is
already removed by returning FALSE at the end of gedit_tab_auto_save().
So it's safer to set auto_save_timeout to 0 before calling
gedit_tab_set_state().
tab: avoid double negation with 'editable'

not_editable = FALSE;
editable = TRUE; /* clearer */