webadmin,userportal: Decouple AspectJ config from gwt-plugin.extraJvmArgs

Some people might run into problems when trying to build WebAdmin
or UserPortal (e.g. OutOfMemoryError) and therefore need to adjust
gwt-maven-plugin JVM args.

Overriding "gwt-plugin.extraJvmArgs" directly is not safe, as it
also contains AspectJ load-time weaving configuration (used to
prevent GWT compiler from pruning certain classes).

This patch includes two changes:

1. Move AspectJ-related configuration out of
   "gwt-plugin.extraJvmArgs", directly into gwt-maven-plugin's
   extraJvmArgs config element.

2. Increase MaxPermSize from 256M to 512M

With this patch, people should be able to override
"gwt-plugin.extraJvmArgs" directly, without having to edit
pom.xml file.

Change-Id: I1761efcec5197121ca97fca3a7f56aa5f1718495
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>