Allow people to selectively remove stock sequences

[jmcwilliams403]

There may be a way to reduce the amount of clashing between custom sequences and stock sequences without needing to have the original file onhand:

A useful feature may be the ability to overwrite previously defined sequences with ones with an output of either an empty string "" or VoidSymbol (an xorg keysym representing "") to remove the original sequence definition from the list, as opposed to its normal behavior of having a new sequence with a literal output of "".

example:

<Multi_key> <1> <slash>     : "⅟" U215F      # FRACTION NUMERATOR ONE
<Multi_key> <1> <slash>     : ""  VoidSymbol # Overwriting to nothing to remove original sequence definition
<Multi_key> <1> <slash> <2> : "½" onehalf    # VULGAR FRACTION ONE HALF

Which would ultimately allow the sequence <Multi_key> <1> <slash> <2> : "½" to work

[dkeenan7]

It shouldn't be necessary to explicitly remove an existing sequence. The mere fact of defining a new sequence that extends an old sequence, should be enough to cause that old sequence to be deactivated (or possibly extended with a <space>). See my comment to #479.