Refactor: Make core a general "3D animation API", move parsing of steps to plugin

This is an idea @bartaz has discussed with me as I became active in this project. I'm writing down this ticket now triggered by #649.

The basic idea is to simplify the core even more, so that core just moves the viewport to given coordinates. Even the parsing of `data-x/y/z` attributes would move out of core to some kind of step-positioning plugin.

As a first pass, I'm going to include the concept of a step as a core object though. It is common for animation software to build animations out of steps, so this seems justified. This will result in a more complex api, not less.

Core objects:

* a step
* an order between steps (array & current index)
* a camera (internally canvas + root objects)

The new API could therefore be:

addStep(element, coordinates, [index]) // default: append. This would be the `css()` part of current `initStep()`
removeStep(index)
getSteps() // return: array
getCurrentStep() // return: {index: ..., el: ..., coordinates: ...}

// Move camera to arbitrary coordinates
// Coordinates is essentially this object: https://github.com/impress/impress.js/blob/master/src/impress.js#L282
setCoordinates(coordinates)
getCoordinates()
panZoom(relativeCoordinates)

// The rest stay same as before, but goto() now becomes setCoordinates(getSteps()[target].coordinates))
goto(target) // as currently, target can be an index, step id
next()
prev()
init()
tear()
lib()

What could move out of core:

* `swipe()` can now move to `touch` plugin and just call `setCoordinates()` repeatedly to animate the swipe.
* Most of `initAllSteps()` and `initStep()` can become 1 or 2 new plugins, which parse DOM attributes and call addStep().
* I believe `addPreInitPlugin()` can then be removed and `rel` and `extras` can become normal plugins triggered by `impress:init`.
* A completely separate question is whether we should adopt the new import/export type modules. impress.js historically was always targeting state of the art JavaScript. But this ticket is valid either way.

Start with src/impress.js, especially the coordinate object around line 282 and the current initStep() and initAllSteps() paths. Map how steps, camera movement, swipe(), and plugins interact before deciding the API boundaries. Done would mean a general step-and-camera core API, with DOM step parsing and swipe behavior moved into plugins while existing navigation remains available.

Written by the indexing model from the issue text.