Changes

Fixed panic when deleting the last tab.
Changed behavior of dropping multiple folders to the application's window: now each of them is processed as a separate tab.
When searching a track in a tree, the program doesn't set table's index to the found match and doesn't interrupt playback order.
Fixed missing .ac3 format association on Windows.
Possibly implemented file associations for Linux.
Fixed random panic related to peak visualizer when playing/switching tracks.
Maybe slightly improved peak visualizer's performance.
Fixed panic when finishing playing the last track in a playlist.

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