Refactor converter runner

Summary:
The logic from the converter runner has been refactored and moved to seperate files.
Additionally some foreach marcos have been refactored and the numberValue of the query gets only
calculated once, instead of being calculated for each matching unit.

Test Plan: The plugin should work as before and the fractional units in https://phabricator.kde.org/D22869 are still supported.

Reviewers: broulik, ngraham, Plasma, sitter

Reviewed By: broulik, Plasma, sitter

Subscribers: sitter, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27166