Don't export slideshow data on MPRIS when no slideshow is happening, fix base64 path generation for dbus

This is an alternative MR to !179 (closed), which fixes a couple of bugs related to MPRIS instead of outright removing it.

• Slideshow data is exported, even when there was no slideshow happening. There's no bug reported about this, but there are a couple of off-hand reports about how weird this feature is. For example, open an image in fullscreen mode, and then back out - Gwenview still shows in MPRIS. Now it only shows when a slideshow is actually happening. Tested on KDE connect too, and it's still possible to remotely start slideshows there too!
• Fixing BUG: 402970 because paths like /tmp/a3o is one of the few paths that generate Base64 strings with + in them, which is actually not a valid DBus path.