Fix segfault: gtk_get_current_event can return null

fix segfault when gtk_get_current_event return null
fix memory leak: gtk_get_current_event return value must be freed

Closes https://github.com/mate-desktop/mate-panel/pull/243
Places menu: don't rely on Caja desktop files

This adds better support for other GVFS-compliant file managers (e.g. nautilus, nemo, pcmanfm).

Closes https://github.com/mate-desktop/mate-panel/pull/242
Merge remote-tracking branch 'upstream/master'