Fix UML-214 Improve "Getting started"

For user installing UML Designer from an existing Eclipse, the
Eclipse default welcome page was extended and content was added in
Overview, Sample, Tutorials and What's new section.
Four cheatsheets were added to explain how to :
- Create a new project
- Create a new diagram
- Import an existing project
- Open the sample example

For people using the UML Designer product, a new welcome page was
added which defines three main sections :
- Getting started
- Find help
- Get involved