feat(*): widget layers

[ruizalexandre]

@johnpryan

I rewrite all my last PR
Compatible with plugins and can combine last way to use layers (with LayerOptions).

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🎸 No breaking changes

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🤖 What's inside?

• Adding MapState.of(context) to get the current MapState
• Adding forceRebuild method on MapState for rebuild all map
• Can use simple widget with FlutterMap now with the new property children
• Fix initState/dispose lifecyle -> called after TileLayer code

📜 More explanation

What the sort in stack with children and layers?
children property is before layers property in stack of Widget.

I want create a simple plugin, how?
Use simple Flutter Widget, example Text("I'm a simple plugin now")

I want create a plugin who can access to mapState, how?
Create a Flutter Widget with MapState.of(context) to get the current mapState.

🆘 Linked issues
Close #535
Close #459
Close #482

[fryette]

Any news on it?

[johnpryan]

Looks good to me, The next step would be to make the options classes go away eventually. For example, MarkerLayerWidget could just have the parameters in MarkerLayerOptions in it's constructor.