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
Update  QBtSession

  Guaranteed to accept strings with native and qt-style separators; guaranteed to use qt-style separators internally, guaranteed to call libtorrent API with native separators
Update WebUI classes

  Guaranteed to show paths with native separators to user
Update SearchEngine classes

  Guaranteed to work with qt-style separators internally; guaranteed to call native OS environment using native separators