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:
{F5811250, size=full}

After:
{F5811247, size=full}

Test Plan:
Open raster image, resize ({key Shift}+{key R}).
Modify width and height values and percentages, with and without
{nav Keep aspect ratio} checked.
  - If {nav 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