Refactor ReactiveTools macros @app, @handlers and @init

[hhaensel]

Currently, the call of the ReactiveTools macros @app, @handlers, @type, @in, etc. violate the no-side-kick principle of macros, i.e. they change the state of the program even when called with @macroexpand.
This generated strange interaction with Revise in one case.

This PR refactors all macros such that the code generated can be viewed by @macroexpanding the calls.

Here's a list of changes:

• @app, @handlers and @init have been refactored and greatly simplified
• @var only supports one API, support of explicit reactive types has been deprecated
• macros @in, @out etc, have been removed, the parsing of the macro expressions is performed by the @handlers macro
• if the @handlers macro is called without a variable section, it will not change the existing app declaration, but only add a handler; when called with a variable section, these variables will overwrite former variables and no longer add them. As before, the resulting handlers function is registered as default handler for that is called by @init
• handling of implicit models is now a thin wrapper on top of explicit models

This PR could be a first step of a potential deprecation of implicit models; explicit models have the large advantage that multiple apps can live in one module and that they can easily share code. Moreover, they are necessary to build multi-app pages, for which another PR is in preparation #299.