Add welcome page in the studio

[dvojtise]

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