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

BUG: 236970

Test Plan:
  # {nav Image} view > enable crop tool (e.g. by using {key shift}+{key
  c}
  # Ensure ratio restrictions work in both {nav Normal} and {nav Advanced}

The ratio settings in {nav Normal} and {nav 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