Send media length to frontend

[nicmr]

Frontends require the track length to display track length and progress bars.

Cannot done for all tracks in advance because the information is only computed by libvlc when the track is played.

Solution: https://docs.rs/vlc-rs/0.3.0/vlc/sys/fn.libvlc_media_player_get_length.html
Unfortunately not yet wrapped by the rust wrapper for libvlc so it's unsafe :/

Remaining issues: cannot compute total playlist / album length

[nicmr]

mostly implemented as of d1743d4
there's still a small issue remaining with the track length being 0 until the page is refreshed, which is probably caused by calling the function to determine the track length before the media has finished loading. will fix in the next days.