craft: portage/extragear/konversation: Build 1.7 branch by default

Summary:
portage/extragear/konversation: Build 1.7 branch by default, remove 1.6

Also allow building git master, but default to 1.7

Test Plan: 1.7 branch actually builds under windows, unlike 1.6 with linking error about winsock libs

Reviewers: KDE Applications, kfunk, vonreth, albertvaka

Reviewed By: kfunk, vonreth

Subscribers: aacid

Tags: Craft, Konversation

Differential Revision: https://phabricator.kde.org/D5458