Fix speedbar help-echo in erc-nickbar-mode

Fix speedbar help-echo in erc-nickbar-mode

* lisp/erc/erc-speedbar.el (erc-speedbar-buttons): "Spoof"
`dframe-help-echo-function' in channel buffers to get around the fact
that we're not using the real speedbar frame.
(erc-speedbar--fmt-sentinel): New variable.
(erc-speedbar-expand-channel): Use cached channel-mode string for mode
item.  Use button chars to mark mode and topic items and move verbose
labels into mouse-hover text.  Also set face for mode and topic.
(erc-speedbar-item-info): Look for `speedbar-token' as a fallback even
when the primary pattern doesn't match.  If the value's contents are
structured like a `format' function call, use them.
(erc-speedbar--emulate-sidebar): Add comment about speedbar setting
everything globally in older Emacsen.  (Bug#63595)