Vaadin version should be configurable in project creation

[Ansku]

I create a lot of projects, and most of those projects need to use older Vaadin versions. Given that the 7.7.3 pom.xml isn't backwards compatible even to the 7.6 branch, it's very annoying to not be able to choose which version to use.

Current workaround options:

• copy over pom.xml contents and .gwt.xml from an existing older project
• use command-line Maven for project creation and import to Eclipse afterwards
• use the legacy Ivy version