Validate children of GestureDetector to check if there is only one native view that can be a target of the gestures

[j-piasecki]

Description

This PR adds a validation step during rendering of GestureDetector that traverses the Fiber tree to check if there is more that one view that could be a candidate to become the gesture target. If there are more the error message will be displayed.
For example, this is not ok:

graph TD;
    Wrap-->NativeView1;
    Wrap-->NativeView2;

but this is fine:

graph TD;
    Wrap-->NativeView1;
    NativeView1-->NativeView2;
    NativeView1-->NativeView3;

Test plan

Tested on the Example app.

[tomekzaw]

👏