Draft: feat(users): Add alpha avatar cropping

Reason for the change

On GNOME, the user is able to crop an image when setting an avatar picture. I thought integrating this into KDE would be beneficial as users wouldn't have to make crops in external applications when setting an avatar.

Screenshots or screen recordings

Before	After

TODO

This is just an early version so I could get some feedback and receive help continuing developing this feature.

Features:

• Improve implementation.
  • I'm not the biggest fan of how I implemented things. The way this works is that once cropped image is set, it creates a cropped version of the image file into /tmp, and uses its URL from there to store the image into the avatar picture. I only did it this way because that's how I knew what to do. I would really appreciate somebody with more expertise here.
• Provide input to make the circle shrink and grow in size.
• Change the visuals. Help might be wanted here, too.
  • The image should be greyed out while the circle should keep the image's lightness. Probably be better if the border was white, too.
  • Move inside handles in the circle and partially cut the handles?
• Prevent the user from moving out of bounds. There's no limit for how far the user can drag the circle.
• Maybe center the circle in the dialog?
• The user has to scroll vertically to view the entire image. Would it be better if the whole image was on a single view?

Bugs:

• Fix: Before applying, the user should be able to select a different image and crop that, and have that reselection shown in the face preview icon above.
• Fix: If the user clicks on anywhere of the handles, it will jump and align its top left corner with the cursor.
• Fix: The top handle doesn't seem like it can keep up with the speed of the mouse.
• Fix: The circle does not readjust itself if the dialog is resized.