webadmin,userportal: Improve TextBoxLabelBase

This patch modifies constructor:

  public TextBoxLabelBase(Renderer<T> renderer) ...

to become:

  public TextBoxLabelBase(Renderer<? super T> renderer) ...

in order to support broader range of renderers, i.e.
renderer for T or any supertype of T, up to Object.

Assuming GWT compiler discards generic type information,
in the worst case, things will blow up on bad method
signature. However, TextBoxLabelBase extends ValueBoxBase
which uses provided renderer like so:

  public void setValue(T value, boolean fireEvents) {
    ...
    setText(renderer.render(value));
    ...
  }

and the above renderer usage should be safe even when
using Renderer<? super T> instead of just Renderer<T>
at runtime.

(Makes me wonder why GWT ValueBoxBase doesn't work
with Renderer<? super T> just like GWT ValueLabel.)

Change-Id: Ie691685ec6e9e1830da1457a0747901ba833be5d
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>