Clarify GtkSettings documentation

Just recommend g_object_set to override settings. The
gtk_settings_set_..._property functions don't really add
any value.
Deprecate some GtkSettings api

The gtk_settings_set_...property functions don't really add
anything over g_object_set(), and we never documented them.
GtkMenuItem: Simplify code

No need to explicitly specify a parser function, it will be
correctly guessed from the property type.
GtkSpinButton: Simplify code

No need to explicitly specify a parser function, it will be
correctly guessed from the property type.
Deprecate gtk_settings_install_property APIs

These are not useful outside GTK+.
GtkToggleButton: Move nonexported API to a private header

This is our standard practice nowadays.
GtkTooltip: Move nonexported API to a private header

This is our standard practice nowadays.