"File Manager" should be renamed "Thunar" everywhere

following #466 (closed) and #482 (closed) having the preference widow named "File Manager Preferences" and the desktop icons (panel and menu) named "File Manager" and "File Manager Settings" makes no sense

everything should be named "Thunar" and "Thunar Settings" or "Thunar Preferences" (not sure why there are 2 naming there)