Fix for moving progress with no data downloaded
[GTKUI] Fix showing the open_folder menuitem

The menuitems would disappear and not reappear if switching between localhost
and remote daemons.
[GTKUI] Suppress unimportant gnome warnings
[#2098] Add function to highlight the torrent folder/file

 * Will show/highlight a file path in system file manager. *nix uses dbus with an xdg_open fallback.
 * [GTKUI] Open Folder still opens the download location but now shows the torrent data file/folder
 * [GTKUI] Files_tab now has a second menu item 'Show' to show a file's location
 * The open_file and show_file functions now use timestamps on *nix so that windows open in front, this fixes recent desktop changes that prevent windows randomly stealing focus.
 * Removed utf8 decode for Windows. All paths should be unicode
string, any resulting errors should be traced to source and corrected.
Flake8 files_tab
Consistent naming of torrent download location to Download Folder

 * Replaced the deprecated use of torrent status save_path with download_location.
 * UIs now use 'Download Folder', replaces 'Save Path', 'Download Path', '... Storage', etc.