[patch request] memory used for undo is never freed

[rmolina]

There is an old report in Debian BTS about memory used growing indefinitely.

Accordingly to the package documentation, undo history is infinite.
Even closing all the draws in qcad, the memory used isn't freed.
This can make memory used by qcad grow indefinitely.

It was reported for 2.0.4.0-1 but is still present on 2.0.5.0-1
Details on Bug#305097. Patches from community are welcome

Thanks