This PR prepares GPv3 for calculating falloff factors when working in multi frame editing mode.
The falloff popover is added to the UI.
A `float multi_frame_falloff` is added to `MutableDrawingInfo`. This is a factor from 0.0 to 1.0f.
A `retrieve_editable_drawings_with_falloff()` is added to the utility functions, which retrieves the falloff factor for each drawing when
multi frame falloff is enabled.
To avoid a copy, the return type of `retrieve_editable_drawings()` and friends is changed from
`Array<MutableDrawingInfo>` to `Vector<MutableDrawingInfo>`.
Pull Request: https://projects.blender.org/blender/blender/pulls/118108
2f0b293da3