Add welcome page in the studio

## Motivation

When starting a fresh standard eclipse, the user arrives at a Welcome page showing the main features available.

It would be useful for newcomers to have entry for GEMOC

## Proposed Solution

Add welcome page support to the studio.
Add an entry opening the Eclipse help system on the "GEMOC Introduction" entry
Also add a link to gemoc.org.

## Proposed Implementation

most probably implemented in https://github.com/eclipse/gemoc-studio/tree/master/gemoc_studio/plugins/org.eclipse.gemoc.gemoc_studio.branding