check empty file path
refactor to use more mpv property observation
* Fix: ignore level check for VDPAU

Apparently, VDPAU just works fine with no level check
* Fix: english locale was ignored
It seems Qt's bug which make loading en_US locale failed always.
Anyway, this should work as a workaround
update translation strings
* New: more informative playback information display
* New: language priority for audio track (#125)
New page for language has been added in preferences and subtitle priority has been unified into that page too.