Put display properties to better use in erc-stamp

Put display properties to better use in erc-stamp

* lisp/erc/erc-log.el (erc-log-filter-function): Add new value
`erc-stamp-prefix-log-filter'.
* lisp/erc/erc-stamp.el (erc-timestamp-use-align-to): Enhance meaning
of option to accept numeric value for dynamically aligned right-hand
stamps.  Use `graphic-display-p' to determine default value even
though, as stated in the manual, terminal Emacs also supports the
"space" display spec.
(erc-stamp-right-margin-width): New option to determine width of right
margin when `erc-stamp--display-margin-mode' is active or
`erc-timestamp-use-align-to' is set to `margin'.
(erc-stamp--display-margin-force): Add new helper function for
`erc-stamp--display-margin-mode'.
(erc-stamp--adjust-right-margin): New function to adjust width of
right margin.
(erc-stamp-prefix-log-filter): New value for `erc-log-filter-function'
compatible with modules that activate
`erc-stamp--display-margin-mode'.
(erc-stamp--display-margin-mode): Add internal minor mode to help
other modules quickly ensure left-right, display-prop-oriented stamps
are showing correctly.  Does not support left-hand-only stamps.
(erc-insert-aligned): Deprecate function and remove from primary
client code path.
(erc-stamp--inherited-props): Add internal constant to hold properties
that should be inherited from any stamp-bearing message being
inserted.
(erc-insert-timestamp-right): Account for new display-related values
of `erc-timestamp-use-align-to'.
* test/lisp/erc/erc-stamp-tests.el (erc-timestamp-use-align-to--nil,
erc-timestamp-use-align-to--t): Adjust spacing for new default
right-hand stamp, `erc-format-timestamp', which lacks a leading space.
(erc-timestamp-use-align-to--integer,
erc-timestamp-use-align-to--margin): New tests.  (Bug#60936.)