Extension to RFrames

[codejaeger]

Is your feature request related to a problem? Please explain.
When specifying a relative frame range it would be would be nice to have an option to specify if the reference frame was the previous object's first or last frame. A particular use case is when objects need to be added to the canvas one after another based on their starting frame.

Describe the solution you'd like
Add the argument from_start::Bool to RFrames to indicate the frame range for this object.
Example:

ball = Object(Rframes(5:10, true), (args...)->somefunc())

Describe alternatives you've considered
A workaround is to create a function to access the starting frame of the desired object in a way like start(obj). The only problem seems to be when the previous object (or a reference to it) is not known.

Additional context
Both the methods were suggested by @Wikunia on Javis' zulip thread.

[Wikunia]

Thanks for opening the issue. Yeah the function might need some extra work. Interested in ideas by @Sov-trotter

[Sov-trotter]

Thanks for the ping @Wikunia . I'll get back here in a few days as I am currently out of town. 😀

[TheCedarPrince]

I think we can close this issue based on the above Zulip discussion. :D