add g_key_file_{get,set}_modified()
@malureau
Submitted by Marc-André Lureau Link to original bug (#681191)
Description
It can be useful to track if the keyfile has been modified, to check if it needs to be saved again.
The API is similar to GtkTextBuffer. The user can manually set the modified flag.
It could eventually be useful to have hook to be informed when the flag is modified.