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.
Updated, also fixed two other bugs (see commit messages).

@b4n:
> 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.

d6dd316 now rewrites the default session to disk if the project creation is cancelled.

> 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?

I think showing it afterwards isn't ideal, but integration into the dialog would be OK. I haven't done that as it seemed awkward enough fixing the behaviours for each use-case, so I'm reluctant to make more changes. At least I've commented the code better for the next person.

> 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.

Default session now gets reloaded when the dialog is cancelled.