build: GWT dependencies cleanups

- Defined GWT maven plugin in root POM plugin management section.
- Defined GWT dependencies in root POM dependency management section.
- Versioned and scopes are inherited from root pom for all sub modules.
- uicommonweb module is self dependent on gwt-user dependency.
- Removed POM changes that are in comments for long time.

Change-Id: Iffb753cd6fb4f669b82a40fd0da5f37d15d18e5f
Signed-off-by: Asaf Shakarchi <asaf@redhat.com>