Added textbox showing RGBA values.

Added textbox showing RGBA values.
Added padding to pixel values.

This commit is related to issue
https://github.com/appleseedhq/appleseed/issues/364

The RGBA values of the current position of the cursor are shown when mouse is moved.
When the cursor is moved outside the image the field is cleared.
Values are currently taken from renderwidget, but this feature could be developed further to take
the values from frame buffer.

Liisa