electrum: 2.7.18 -> 2.8.1

See https://github.com/spesmilo/electrum/blob/master/RELEASE-NOTES

Also
- patch .desktop file to include full path to electrum
- add dep on pysocks
- remove dep on pyasn; per the changelog, it has not been used since v2.1
- replace dep on slowaes with pyaes