Allow layer click-through

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About this PR

Added "clickthrough" boolean property. Makes an AgsWindow not block mouse input for windows below if set to true
Example:

• Screen corners in Aylur's setup or my setup would benefit from not being clickable. I think this image explains it
  

Does it need extra work?

Yes
I've only figured out how to make clickthrough work
I haven't made it work the other way round and the getter func, and I'd really appreciate pointers on how to do these.

Reference

• Fix: passthrough click events Kljunas2/activate-linux#11
• https://docs.gtk.org/gtk3/method.Widget.input_shape_combine_region.html

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Currently, we can get the prop and set it to true, but not to false.
I can't find any docs, and I'm not sure if this is even possible in GJS.
I'll let @Aylur decide whether this is acceptable or not.