Split Image Size into Width/Height

Summary:
This patch splits the single property "Image Size" into "Width" and "Height", providing more fine-tuned control for power users.

FEATURE: 374559

Test Plan:
- Sorting works correctly
- No real change, only exposed differently

Reviewers: #dolphin, elvisangelaccio, ngraham

Reviewed By: #dolphin, elvisangelaccio, ngraham

Subscribers: ngraham, elvisangelaccio

Differential Revision: https://phabricator.kde.org/D11816