Rework fsutils class to only use Qt-style separators

  fsutils methods are guaranteed to accept strings with both native and qt-style separators and return strings with qt-style separators where appropriate
Update Preferences class

  Preferences class setters are guaranteed to accept string with both native and qt-style separators, getters are guaranteed to return string with qt-style separators
Update options UI

  Guaranteed to work with strings with qt-style separators internally, shows string with native separators to the user
Update QTorrentHandle

  Guaranteed to accept strings with both native and qt-style separators; guaranteed to use native separators with libtorrent API