Add ability to constrain crop tool to current image ratio

Summary:
The crop tool was lacking the ability to choose the ratio of the current image.
This patch adds:

• Checkbox in Normal to toggle between no restriction (default), and restrict to the current image ratio
• "Current Image" item to the Ratio combobox in Advanced
• "Unrestricted" item to the Ratio combobox in Advanced (default, functionally no change from before)
• Clear button to the Ratio combobox in Advanced, to help make it clear the user can enter a custom ratio

BUG: 236970

Test Plan:

1. Image view > enable crop tool (e.g. by using ⇧+c
2. Ensure ratio restrictions work in both Normal and Advanced

The ratio settings in Normal and Advanced are independant of
one another, in prep for future patch which will have these remember the
chosen setting.

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: muhlenpfordt

Differential Revision: https://phabricator.kde.org/D11202