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.
I didn't look deeply at this PR, but one thing that's always bothered me with the new project dialog is how it asks for confirmation to close the current project before you've actually done anything, and then if you proceed to cancel creation of the new project, there's no project open. IMO, it should just show the new project dialog without any intervention, and if a user fills everything in and clicks "Create" button, then just silently save/close the existing project and open the new one, or if the user clicks "Cancel" button, just close the dialog and nothing needs to be done.

Not sure it's totally relevant here, but something that's always bothered my UX-sensitibilities and could perhaps be worked into the solution :)