Bugfix (Windows): Bindata wallpapers weren't resolving

* The Windows build of v0.6.0 couldn't load embedded wallpapers such as
  legal.png when asked, but could load the hard-coded default.png
* Root cause was the filesystem.FindFile() checking for path separators
  in the filepath, and on Win32 this is \ but the internal wallpaper
  paths use /

pkg/filesystem/filesystem.go

@@ -87,7 +87,8 @@ func FindFile(filename string) (string, error) {
 	var filetype string
 
 	// If the filename has path separators, return it as-is.
-	if strings.ContainsRune(filename, filepath.Separator) {
+	if strings.ContainsRune(filename, filepath.Separator) ||
+		strings.ContainsRune(filename, '/') {
 		return filename, nil
 	}