Saving any scans after first scan overwrites the file (Regression 1.x to 2.x)

[Dmitri Amariei]

When saving a scan, it will ask you for the save location and file name, however if you click save after scanning another page, it will not ask for a new save but will overwrite the previously given file.
Save As... still functions as intended.

This is a regression from 1.x to 2.x

Steps to reproduce:
1) Start simple-scan
2) Click Scan
3) Click Save, select name and location
4) Click Scan again
5) Click Save (no dialog appears, first scan is overwritten)

Original Bug Report:
https://bugs.launchpad.net/ubuntu/+source/simple-scan/+bug/688241