* Blend group layers on `DrawingAlgos.blend_layers()`
* Support group layer blending on the canvas
* Allow editing of group layer properties
* Fix issues with group layer blending in canvas, and unite common code
* Group layers can now be used as clipping masks
* Make move tool preview work on child layers
* Change OffsetImage's `blend_layers()` to support group layer blending
* Support group layer blending in the canvas preview
* Fix layer blending mode, clipping mask opacity and cel opacity not being updated automatically if the layer/cel changed is not selected
* Add a pass through blending mode to layer groups
Fingers crossed that no bugs were introduced
* Fix issue with layers that belong to pass through groups not updating their textures on the canvas automatically on undo
A very simple implementation, not as complex as something like #768 yet, but it can be done in the future.
The main current limitation is that it doesn't work with group layers as of right now.
