Properly handle existing session when creating new project

Ask whether to adopt the existing session documents.

Currently, when a new project is closed, Geany reloads the default session from the point of project creation. This is very confusing if the new project keeps the same session files, then on closing it they are reopened. It's better to ask the user what they intend.
Hum, yes it might be a big deal if the users answers "no" to that question, and then cancels the project creation, as this would result in an empty session.