Added screen reader support, if backdropEnabled is true.

[nilsreichardt]

Description

As I showed in #168 backdropEnabled == true breaks the app for screen readers. This is very critical...

What was the problem?

The problem was, that GestureDetector already have a Semantics widget included. The backdrop takes up all the space. Because of this a screen reader tried every time to read the backdrop instead of the real content of the app.

Solution

The solution is to use ExcludeSemantics, which ignores all Semantics in the widget tree.

Try it out

Until this PR isn't merged, you can use this in your pubspec.yaml:

sliding_up_panel: 
   git: 
     url: https://github.com/AndroidNils/sliding_up_panel 
     ref: screen-reader-support

Closes #168