Part of https://github.com/harfbuzz/harfbuzz/issues/1682

This reverts commit d219f899.

API change: Un-deprecate font kern API.

Part of https://github.com/harfbuzz/harfbuzz/issues/1682

We should document that this API is only necessary to hook up
non-OpenType kerning.  hb-ot-font will continue to NOT implement them.

New version has axis flags.

New API:
+hb_ot_var_axis_info_t
+hb_ot_var_find_axis_info()
+hb_ot_var_get_axis_infos()

Deprecated API:
-HB_OT_VAR_NO_AXIS_INDEX
-hb_ot_var_axis_t
-hb_ot_var_find_axis()
-hb_ot_var_get_axes()

I think I like the uniform "hb-*" more.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1260

`hb_ot_tags` replaces `hb_ot_tags_from_script` and
`hb_ot_tag_from_language`.

`hb_ot_layout_table_select_script` replaces
`hb_ot_layout_table_choose_script`.

`hb_ot_layout_script_select_language` replaces
`hb_ot_layout_script_find_language`.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1232

New API:
- hb_font_get_nominal_glyph_func_t
- hb_font_get_variation_glyph_func_t
- hb_font_funcs_set_nominal_glyph_func()
- hb_font_funcs_set_variation_glyph_func()
- hb_font_get_nominal_glyph()
- hb_font_get_variation_glyph()

Deprecated API:
- hb_font_get_glyph_func_t
- hb_font_funcs_set_glyph_func()

Clients that implement their own font-funcs are encouraged to replace
their get_glyph() implementation with a get_nominal_glyph() and
get_variation_glyph() pair.  The variation version can assume that
variation_selector argument is not zero.

Add deprecated alias for old name.

Makes documentation / introspection easier.

API additions:

	hb_segment_properties_t
	HB_SEGMENT_PROPERTIES_DEFAULT
	hb_segment_properties_equal()
	hb_segment_properties_hash()

	hb_buffer_set_segment_properties()
	hb_buffer_get_segment_properties()

	hb_ot_layout_glyph_class_t

	hb_shape_plan_t
	hb_shape_plan_create()
	hb_shape_plan_create_cached()
	hb_shape_plan_get_empty()
	hb_shape_plan_reference()
	hb_shape_plan_destroy()
	hb_shape_plan_set_user_data()
	hb_shape_plan_get_user_data()
	hb_shape_plan_execute()

	hb_ot_shape_plan_collect_lookups()

API changes:

	Rename hb_ot_layout_feature_get_lookup_indexes() to
	hb_ot_layout_feature_get_lookups().

New header file:

	hb-shape-plan.h

And a bunch of prototyped but not implemented stuff.  Coming soon.
(Tests fail because of the prototypes right now.)

Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly.  This makes it easier to
refactor headers later on without breaking compatibility.

Step version up to 0.5.0.

Also, fix to pass "make distcheck"

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.