Add initialHeight Functionality

[ABausG]

I was missing a way to set the starting height of the Panel to a value between PanelState.CLOSED and PanelState.OPEN

I added a field called initialHeight which defaults to 0.0 and is used instead of a Hardcoded 0.0 value as the initial value of the AnimationController.

This is just a first proposition on how to use the value. Another Idea I had was to default this to null and check if initialHeight was set and fallback to defaultPanelState if it's null.

Let me know what you think

[aBuder]

It would be great to have this

[JaffaKetchup]

Hello, Is there any news on this? This would really help my project. Thanks!

EDIT: Meanwhile, if anyone else wants to use this, replace the existing field in your pubspec.yaml file to:

sliding_up_panel:
    git:
      url: https://github.com/ABausG/sliding_up_panel.git
      ref: initial-position

This will get the forked version, so you can use the features in this fork before it's published.