Adds a PopupPanel.isShowing() method, implements PopupPanel.isVisible() to match PopupPanel.setVisible(), and fully defines the difference between setVisible()/show()/hide().  This patch also removes the ChangeHandler from TextBoxBase in favor of the ValueChangeHandler.

Patch by: jlabanca
Review by: ecc (desk)
Issue: 1665, 3233



git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4412 8db76d5a-ed1c-0410-87a9-c151d255dfc7