Add $u to Custom Menu variables (#891)

`$u` is replaced with the 4 digit hex Unicode codepoint
value of the current character shown in the status bar,
or the empty string if none.

The following will look it up at unicodeplus.com and
fileformat.info:
`https://www.unicodeplus.com/U+$u`
`https://www.fileformat.info/info/unicode/char/$u/index.htm`

Add `$u` to help string

Author: windymilla