As discussed offline, this refactors the Filters.msg_in filter to work as __call__ of Filters.text, thus closing #1625
I encountered two (and a half) obstacles:

• BaseFilters __call__ is used to perform the update checks, i.e. it executes filter(). To allow for Filters.text({'foo', 'bar'}) anyway, Filters.texts __call__ now checks, if the argument is of type Update. If it's not, it returns an instance of a subclass that works like the former msg_in. An alternative way would be to do something like

  def __call__(self, update=None, iterable=None):
      if update:
          self.filter(update)
      else:
          return _TextIterable(iterable)
  

  However calling Filters.text(iterable={'foo', 'bar'}) seemed neither convenient nor intuitive for me. Also, usually you won't have to call Filter.text(update) manually anyway …

• To recreate the caption filter functionality, I needed to add the base class Filters.caption that allows only messages that have a caption. Including caption filtering into Filters.text like

  Filters.text({'foo', 'bar'}, caption=True)
  

  didn't seem very natural to me …
  If you feel like Filters.caption is unneeded, we can just remove it. The main usecase for this PR is Filters.text(buttons) anyway …

• I wasn't very original in naming the subclasses (_TextIterable and _CaptionIterable). If you have a better naming idea, please tell :)