impl seekbar

[skentagon]

Implemented a seek bar using QSlider.
Note: This has not been tested in a VR environment, as I do not have access to VR hardware.

[marlam]

The long-term plan for Bino is to implement a kind of overlay GUI, similar to other video players.
This would be displayed in the video area and thus also work in VR mode. In VR, pointing a controller and pulling the trigger would replace moving the mouse and clicking.

This overlay GUI would include the main controls, including a seek bar.

I am therefore hesitant to merge this - for one, the patch seems to disable seeking via keyboard shortcuts, but more importantly, I think in the long run the overlay GUI would make more sense. On the other hand, I don't know when I will get around to implement it.