webadmin,userportal: Fix GWT DevMode refresh issue

GWT SDK version 2.5.1 + gwt-maven-plugin version 2.5.1 combo
has an issue where GUI code change (modifying *.java file)
isn't reflected into (running) GWT DevMode debug session:

  https://jira.codehaus.org/browse/MGWT-350

In practice, this means developer has to restart GWT DevMode
each time he (she) makes GUI code change, which is not good.

This patch modifies 'gwtdev' Maven build profile to work
around this issue by using gwt-maven-plugin version 2.4.0
instead of (GWT SDK based) version 2.5.1.

Note that 'gwtdev' profile is used only for GWT debugging
purposes, therefore this patch has no impact on regular
GWT build.

Change-Id: I831d0b2ce203bea3a4ffa1b42c27a3d0c300c7a1
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>