Prepare to use Arc's status colours in the revision drop-down list

Arcanist uses green, magenta and red to distinguish the user's open
differential revision by status: accepted vs. "needs review" vs. "needs
revision" respectively.
This uses that same convention for the Phabricator plugin drop-down list
of open revisions that can be updated - in Qt versions that (will) support
controlling text colour through a ComboBox model.

Differential Revision: https://phabricator.kde.org/D13933