[Media Controller] Explicitly set slider "to" before setting "value"
Slider refuses to set a value beyond its end, make sure "to" is up-to-date before we attempt to set "value".
Position changed may be emitted before Length change was emitted.
Fixes media controller position not being up to date on first open.
Started playing a media file, paused it somewhere in the middle, restarted plasmashell, opened media controller. Position was correct on first opening, before it opened at zero.