Add the searchEnabled flag to getSearchAction

[dineshba]

This is to fix the issue #25.

Please have a look and provide the feedbacks. Thanks.

[dineshba]

I have added an example to test it. Please refer https://github.com/dineshba/flutter-search-bar/commit/c1fee7a0aa646b4887207e74d9b6af2eee9a605b

[dineshba]

@ArcticZeroo When you are available please have a look at it

[ArcticZeroo]

I think we may want to implement it a different way. Here's my thoughts on the implementation:

• We add another ValueNotifier for search being enabled (ideally a private one which is controlled by a getter and setter property)
• The SearchAction is a widget whose opacity is controlled by the value of the value notifier

When the developer changes the value of the value notifier, the opacity can be updated pretty easily (though it may require wrapping the component). It seems heavy-handed to me to require the developer to know if the search is enabled when requesting the search action.

Realistically though this may be a failure of how I wrote the search bar in the first place, and I intend to eventually replace it with a single stateful widget.

Thoughts?