create variable backdrop

[lastmeta]

Describe The Task

For various pages we have a larger header on the backlayer. This poses a challenge since the "page" is confined to the front layer.

I've thought of two ways to handle this.

1. we could make a backlayer widget which wraps the nav drawer and whatever else we might need (balance/send/transaction) and depending on the route it could show what we want it to show. We'd also have to make the front layer drop a variable amount.
2. we could make the main page have a regular scaffold on it rather than a backdropscaffold. then on individual pages we could make a backdrop scaffold without an app bar.

We've nested scaffolds before without issue, but it is bad practice, and we're already setup for 1 so I'm going to try that one first.

Additional Context

this structure will support #110

[lastmeta]

option one seems pretty manageable, I can automatically reveal a variable amount of the back layer on going to any page, and I can display anything on the backlayer I want per page. But, there's something we'll have to fix -

Backdrop.of(components.navigator.routeContext!).revealBackLayer();

this funciton applies a scrim to the front layer and if you click the front layer it automatically hides the back layer. This is not what we want on the Send, or Transactions pages. we only want that on the home page where we're hiding the nav drawer. soo... I'll probably have to make a sister function to that one without the gesture detector or the scrim. Once I can do that option number 1 will be proven to be sufficient for our needs I think.

And besides, even if we went with option 2, we'd have to make that ability anyway.

[lastmeta]

fluttercommunity/backdrop#117