Show a warning when an edited .ale source file is not in the environment

[echebbi]

Motivation

It may happen that the user edits an .ale source file that is not part of the project's ALE environment. The direct consequence is that the file is not taken into account by the Xtext validators (this is because we don't actually parse the edited file but parse all files that are part of the ALE environment). This may confuse users who would expect their source file to be validated.

Proposition

Show a warning at the top of the editor when the edited file is not part of the environment and provide a quick fix to add it.