No "project already exists" dialog when saving

[Berg]

1) Ubuntu 11.04
2) PPA
3) 1.3.1

Steps to reproduce:
1) Start OpenShot.
2) Click File->New Project and choose x as the project name.
3) Restart OpenShot.
4) Click File-> New Project and choose x as the project name.
5) Note that no "Are you sure you want to overwrite the existing project with the same name?" dialog is shown, so your old project is overwritten. This also works with File->Save when the project hasn't been saved yet, as well as File->Save as.

What should happen:
An "are you sure" dialog should be shown.

Patch below.

[Andy Finch (fincha)]

Thanks for the patch Emil.