[SF#838]Don't use FM_FILE_LAUNCHER_EXEC_IN_TERMINAL if quick_exec option is true.

That choice is useless in most of cases anyway and user who requested the
quick_exec option in the first place wants it to omit terminal.
A bit precise handling of context menu options in Places items menu.

It is better to check g_mount_can_unmount() and disable 'Unmount' option.
For non-volume mounts the 'Eject' option should be explicitly removed.