Support right-align in mode-line

Support right-align in mode-line

* lisp/bindings.el (mode-line-right-align-edge):  New custom
variable, controls where `mode-line-format-right-align' should
align to.
(mode-line-format-right-align):  New function.  If the symbol
`mode-line-format-right-align' appears in `mode-line-format',
then return return a padding string which aligns everything
after that symbol to the right.  Padding width is altered with
the display property and depends on the value of
`mode-line-right-align-edge'.
(mode-line-format-right-align):  New variable.  Convenience
definition for including right alignment in `mode-line-format'.
* doc/lispref/modes.texi (Mode Line Variables):  Document new
alignment functionality and user option.  (Bug#62606)