webadmin,userportal: Improve form-based sub tabs

This patch improves existing form-based sub tab implementation:

1. FormBuilder/AbstractFormPanel no longer clears and re-attaches
   form item widgets (name/value) to detail views on each update.
   Instead, it merely updates existing form items.

2. CSS styles improved so that AbstractFormPanel.setColumnsWidth()
   is now redundant. HorizontalPanel that wraps all detail views
   ensures that all columns are equally wide, allowing consistent
   look & feel for all form-based sub tabs.

3. FormBuilder.showForm() renamed to update() since it now just
   updates all form items. It also makes sure that the property
   change listener is removed from the old model.

4. In FormItem, instead of overriding UIObject.isVisible(), we
   use getIsAvailable() method for controlling form item
   availability (visibility) within the detail view.

5. Updated related classes due to FormItem/FormBuilder API change.

Change-Id: I8b2f2577b1b488d5514dc05c2590a2bb53fcfe4f
Bug-Url: https://bugzilla.redhat.com/915904
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>