Renaming does not check for valid characters; breaks from there

[Vadim Peretokin]

Steps to reproduce:

- Take a screenshot
- Rename it to something that contains illegal characters (like a forward, backward slash).
- Now things get awry...
- Shutter tells you that the file wasnt found on disk
- If you cancel the saving, the tab will close, but the thumbnail will still be in the session
- Double-clicking on the thumbnail will cause this:
*** unhandled exception in callback:
*** undef is not of type Gtk2::Widget at ./shutter line 2152.
*** ignoring at ./shutter line 1376.

- what should be done instead:
- as the user is typing the characters, check for illegal ones. If one is found, disable the 'Rename' button, and show a text in small letters that the illegal characters (list which are used) aren't allowed to be in the filename.