Add handling for application/x-zerosize mimetype

As per glib#2777 (closed) GLib will soon return mimetype 'application/x-zerosize' for zero-size files instead of 'text/plain'. These changes, I think, will let gedit handle 'application/x-zerosize' same as 'text/plain'.

On Windows the mimetype is 'text' instead of 'text/plain' for plain text files. Because of that I'm uncertain how 'application/x-zerosize' should be handled on Windows and I've not touched Windows specific code. These two plugins have Windows specific code and may need additional changes:

• plugins/filebrowser/gedit-file-browser-store.c#L1946
• plugins/quickopen/quickopen/popup.py#L166

Fixes #528 (closed).