webadmin,userportal: Support generic TextBoxLabel widget

1. Created TextBoxLabelBase (abstract class) with generic
   type parameter and common behavior taken from original
   TextBoxLabel widget

2. Modified TextBoxLabel to extend TextBoxLabelBase<String>
   with String-specific behavior taken from original
   TextBoxLabel widget

This patch is meant to support creating custom (type-specific)
TextBoxLabel widgets to replace any ValueLabel widgets within
"General" sub tab views to achieve consistent look & feel.

For example, this patch should support replacing this:

  public class VersionLabel
      extends ValueLabel<RpmVersion> {

    public VersionLabel() {
      super(new VersionRenderer());
    }

  }

with this:

  public class VersionTextBoxLabel
      extends TextBoxLabelBase<RpmVersion> {

    public VersionTextBoxLabel() {
      super(new VersionRenderer());
    }

  }

Change-Id: I728f7d6fdc74523ac397225c1c5833cb84f6eef9
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>