Add support for the min-width display property

Add support for the min-width display property

* doc/lispref/display.texi (Display Property): Document
get-display-property.
(Other Display Specs): Document min-width property.

* src/dispextern.h (struct it): Add fields for min-width handling.

* src/xdisp.c (find_display_property, get_display_property): New
helper functions.
(display_min_width): Insert stretch glyphs based on the min width.
(Fget_display_property): New defun.
(handle_display_prop): Handle min-width ends.
(handle_single_display_spec): Handle min-width starts.