plain madx files

It is now possible to use multiple independent `Madx` instances, see [Redesign cpymad backend](https://github.com/pymad/pymad/pull/44).
This allows to process arbitrary madx files. A corresponding view has to be developed. It can probably act as a base class for the current `MadLineView` which is specialized for models.
Possibly, even elements can be shown, if madx files are preprocessed by [madseq](https://github.com/coldfix/madseq) or if implementing [inspect sequence from memory](https://github.com/pymad/pymad/issues/38).

In fact, this is a major concern and should be implemented ASAP, since it makes it easier to use custom .madx files ad-hoc without prior creation of model definitions.