Simplify building the form

[OndrejKunc]

Right now, the form initialization can be quite confusing for new users. They need to care about objects like FormManagerBuilder, FormManager or FormRendererService.

We can wrap those objects into a set of helper widgets that will hide those implementation details. Also, by using InheritedWidget we can make still objects like FormManager accessible after it is created.

[OndrejKunc]

As a result of this issue, 3 new widgets were implemented

• FormProvider - Inherited widget providing FormManager
• ParsedFormProvider - Built on top of FormProvider but also takes care of parsing the form
• FormRenderer - Widget taking FormManager and list of renderers to render form on the screen.

Also FormManager was changed to abstract class and I added a set of inherited classes that take care of building the form without the need to manually create FormBuilder.

For the more API description please read updated Readme.md on the main page https://github.com/OndrejKunc/flutter_dynamic_forms/blob/master/README.md or see the updated examples in the flutter_dynamic_forms_components/example