Resolve #499 and add tests

[djyotta]

Resolves #499 - FloatEdit mangles decimals

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment - not familiar with this, sorry

Description:

Fixes FloatEdit mangling decimals.

Uses Decimal library instead of manipulating the string representation in urwid.

[penguinolog]

re-validated manually in addition to tox