add: placeholderWidget parameter

[LeGoffMael]

• new placeholderWidget parameter to display a widget when the image is null
• new onLoading function parameter to be aware when the crop area is ready to be used

also fixed an issue where switching image causes an exception (#85)

════════ Exception caught by image resource service ════════════════════════════
The following assertion was thrown by a synchronously-called image listener:
Cannot get size during build.

The size of this render object has not yet been determined because the framework is still in the process of building widgets, which means the render tree for this frame has not yet been determined. The size getter should only be called from paint callbacks or interaction event handlers (e.g. gesture callbacks).

example

here is a video showing how a thumbnail and a loader can be used as placeholder

RPReplay_Final1671184314.mov