Use current image for line profile

Originated from:

```
---------- Forwarded message ----------
From: Nadav Horesh <nadavh.horesh@gmail.com>
Date: Tue, May 27, 2014 at 3:47 AM
Subject: Updating LinePorofile on image changed
To: scikit-image@googlegroups.com


Hi,

 I built an application based on skimage viewer some image filter plugins and a LineProfile. In this application I want to investigate the influence of the filters (tuned by some attached sliders) via the LineProfile tool. My problem is that the tool sees only the original image, and not those generated (and displayed in viewer window) by the filters.

 How can I make the LineProfile tool relate to the modified image?

  Thanks,

    Nadav.
```

Now it will default to the current image instead of the original.  Do we want to add the option of which one to use?

I used the following to test it:

```python
from skimage import data
from skimage.filter.rank import median
from skimage.morphology import disk

from skimage.viewer import ImageViewer
from skimage.viewer.widgets import Slider, OKCancelButtons, SaveButtons
from skimage.viewer.plugins.base import Plugin

def median_filter(image, radius):
    return median(image, selem=disk(radius))

image = data.coins()
viewer = ImageViewer(image)

plugin = Plugin(image_filter=median_filter)
plugin += Slider('radius', 2, 10, value_type='int')
plugin += SaveButtons()
plugin += OKCancelButtons()

viewer += plugin
from skimage.viewer.plugins import LineProfile
viewer += LineProfile()
viewer.show()
```

It does not update the line profile when the image changes, but you can see a difference with a small radius vs a large radius filter after moving one of the endpoints slightly.