Allow customization of the BasePackage and Model Directory for the GenModel

[CamilleLetavernier]

Original issue: eclipsesource/ecore-glsp#129

The default genmodel is sufficient to generate some java code, but in most cases, the user wants to at least customize the following fields:

• base package (Otherwise, the generated packages match the Ecore package name, which can't include segments)
• target project / source folder

These values should be additional, optional parameters in the GenModel creation command (Until we get a proper genmodel editor; or at least a simple properties page for it)

[CamilleLetavernier]

These values can be set (with input fields) when creating the GenModel; so this ticket can be closed. In the future, we probably want to provide a Tree Editor for GenModel, to support all parameters, and to modify them after the creation of the GenModel.