Horizontal layout

[hanskokx]

I would like to see a horizontal layout, wherein the backLayer can be positioned on either the left or right side of the screen. Upon revealing the backLayer, the frontLayer would slide to the side. This would allow me to use the same code for both wide and narrow layouts, rather than trying to switch controls. I'm looking into NavigationRail as a possible solution for my wide layouts, but this isn't ideal.

[daadu]

#17 #34 are the issues for the particular feature. will be working on it after closing #5

[daadu]

@hanskokx Most of v1.0 is closed (except proper documentation), you can go ahead and start working on it in backwards-compatible way. Ideally, we should be implementing it with BackdropScaffold itself, can you design the APIs to achieve (horizontal layout or NavigationRail) it?

[hanskokx]

I have been pondering how best to achieve this, and I would agree that `NavigationRail` should be considered.  With the current method of using a standard `AppBar`, there doesn't appear to be an elegant method to conceal the `BackPage` header and expose the left (or right) side of the screen.  Perhaps `NavigationRail` will solve this, as it is adaptive.  If/when I get some time, I'll experiment and see if I can come up with something.