Add ability to resize images based on percentage

Summary:
Adds Percentage width/height fields to the image resize dialog,
and ensures all fields appropriately adjust automatically.

BUG: 207802
FIXED-IN: 18.08.0

Before:

After:

Test Plan:
Open raster image, resize (⇧+R).
Modify width and height values and percentages, with and without
Keep aspect ratio checked.

• If Keep aspect ratio checked, percentages should always be exactly the same
• Values should be consistent, no matter which value is manually modified.

Confirming the resize should work as normal.

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx, ngraham, muhlenpfordt

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D12300