Add timeline / seeking

[jmbreuer]

Hi,

first of all thanks for that great little project!

I love using this together with KDE Connect to use my phone as a quick & dirty remote for VLC (on Windows, under Linux this already immediately works thanks to MPRIS).

Having timeline seeking / that 'skip back 10 seconds' button turned out to be desirous enough for me to sit down a couple of hours and code up these additions. 😉

This is not so much intended as a ready-to-go pull request, but rather to let everyone know that my fork exists, and what additional features it might offer. I'm looking into volume control next, that's a bit cumbersome with KDE Connect and VLC under Windows, but again works smoothly under Linux out-of-the-box.