Proof of concept new style specs

Hi again all,

Here's a little proof of concept for how I would like to see style specs working. The big idea here is to
* Get rid of `all_the_styles` and all the `behaves like` things that were added [here](https://github.com/shoes/shoes4/blob/master/spec/shoes/shared_examples/style.rb).
* Test the 5 setter methods and 2 getter methods for styles described [here](https://github.com/shoes/shoes4/pull/683#issuecomment-43389269). In this PR that looks like this:
  * **Setters**
    - **During creation of element** - tested in element tests no changes in this PR
    - **Via element-setter** - newly added tests iterate through each supported style and check that the element responds to a setter and that `update_style` gets called.
    - **Via style method** - small modifications to existing tests for this
    - **Element-level setting** - newly added tests (discussion in line)
    - **App-level setting** - tested on app no edits
  * **Getters**
    - **`style`** - small modifications to existing tests
    - **method** - newly added tests iterate through each supported style and check that the element responds to the getter and that `update_style` does not get called

* I hope that this will generate some discussion to make sure everything is getting tested that we can think of (well . . . or that we can think of how to test) before putting #711 to rest.

* I've only applied these tests to arc (and commented them out temporarily on the others) to keep things simple, but I'll add `it_behaves_like "object with style"` to all the elements from #711 before this is submitted.

Sound good?
These args are one of the biggest questions I have about this new approach. I like that I can test `style(Shoes::Element, stroke: orange)` more directly, but I'm not sure I like the way this part of it works. Any suggestions?