Move tile URLs into own statement

This is part of an effort to make the URL configuration and the control
package more readable and maintainable. I double checked, that all
catmaid.control namespace imports removed, where not in use in other
places.

This commit relates to issue #570.
Add missing getOpacity() method to text label layer

Currently, every layer has to implement a getOpacity and setOpacity
method. The text tile layer was missing the former. This relates to
issue #585.