tab: remove comments explaining standard GObject code

The comments clutter the code and make them less readable. For someone
unfamiliar with GObject the best is to first learn GObject.
tab: remove unused #include's
tab: simplify install_auto_save_timeout_if_needed()

The return value was never used.
tab: remove install_auto_save_timeout()

Use only install_auto_save_timeout_if_needed(). It simplifies the code.
tab: simplify auto save timeout removing
tab: update_auto_save_timeout() in gedit_tab_set_state()

The function adds or removes the timeout according to the state and
other properties. The function is called in gedit_tab_set_state(), so
other GeditTab features don't need to explicitely remove/add the auto
save timeout a bit everywhere in the code...
tab: use g_signal_connect_object() for the print job
tab: print debug progress only once

And fix the FORMAT.
tab: don't set GEDIT_TAB_KEY on the view

The GEDIT_TAB_KEY is set only on the GeditDocument. For the view we can
anyway get its GeditDocument easily with gtk_text_view_get_buffer().
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 */
tab: make code of document_loading() and document_saving() clearer

No, the time is not given by a certain extra-terrestrial.
tab: make the code of document_loaded() clearer
tab: one more small cleanup in document_loaded()

Have this commit separated from the other cleanups, for git bisect. But
normally gedit_tab_get_document() returns the same document.
tab: some cleanup
snippets: use new private definition for gobject templates
Use the new Gtk style class for message dialogs
Release 3.13.1