Properly handle existing session when creating new project

Ask whether to adopt the existing session documents.

Also fixes the existing bug of not reloading the default session when
an existing project is open and a new project is cancelled.
Properly close abandoned new project that couldn't be written

Also rewrite the default session in case it was cleared.
Fix GLib warning 'app->project == NULL failed'

Occurred on Project->New with an existing project when closing an
unsaved file was cancelled.
Merge pull request #315 from ntrel/move-session

Properly handle existing session when creating new project