flutter_package / fluttertpc_get

- Change GetPageBuilder to WidgetCallback typedef, so we can use it outside Get and the name makes more sense.
- Added isDesktop/isMobile
- Added GetPlatform (web) getters to detect the OS it runs (using navigator UA)
- Added nil extension (can be useless now), to enforce the null type safety in JS for old dart2js versions (was undefined)

- fixed types in equality operator, added hashCode override to return rx.value.

docs changes

Update README-es.md

- fixed code sample in simple_builder.dart
- Added docs for ValueBuilder and ObxValue

General cleanup, more docs.

- Added a Navigation. methods references in docs, for easier understanding of the names.

- improved comments.

Simple bool notation

[Docs] Document how to update observable class

- Added SingleGetTickerProviderMixin to rx_interface.dart, usage like SingleTickerProviderMixin in a StatefulWidget, but only valid for DisposableInterfaces.

- improved API and docs.

- cleanup more comments.
- promote most classes in parse_route.dart (except ParseRouteTree) to be private. I still believe we should use Uri to parse the "url".
- Added _RxImpl::refresh() in rx_impl.dart and added some comments/samples.
- Some cleanup in rx_interface.dart, added more comments, missing types and move vars to the top.

GetInstance cleanup

- Fixes issue #521. Still needs to be tested, is a partial solution and i believe parse_route.dart requires a much deeper cleanup.
- Added a new BindingsBuilder(), to simplify the Bindings generation without the need to use a custom class.
- Modified several operators in _RxImpl and subclasses. rx.toString() solves to rx.value.toString(), and in all Rx >>= works as assignment, also specific operator overloads were added to RX versions of List, String, double, num, int.

improvement of logging, replaced print method for developer.log

- Added a fix (a little bit hacky) on DisposableInterface (rx_interface.dart), to prevent method overriding in subclasses, preventing devs to break the lifecycle of Controllers. This compensates the lack of `final` methods in Dart, or the inability to limit the scope of methods to internal packages.

- Cleanup log messages to have a more consistent format (still work to be done there).