adapt to fetchpo changes and use vars for target directories to dry code

Summary:

• fetchpo now expects the output dirs as named arguments
• new argument for poqm directories where releaseme will put _qt.po files
• variables for both po/ and poqm/ to not repeat the paths all over the place

Reviewers: apol

Reviewed By: apol

Subscribers: Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D5525