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.
Having this before actual project creation means that if the project creation fails (error or cancellation), the default session is cleared.  This is not so much of a big deal as it is supposed to be written back when the instance quits, but it might be annoying if something leads to a crash after and the previous session is lost.

Maybe the whole question dialog should either be integrated as an option to the New Project dialog, or simply shown after the project was created?