queue2: Distinguish between buffering percentage and buffering level

queue2: Distinguish between buffering percentage and buffering level

To make the code clearer, and to facilitate future improvements, introduce
a distinction between the buffering level and the buffering percentage.

Buffering level: the queue's current fill level. The low/high watermarks
are in this range.

Buffering percentage: percentage relative to the low/high watermarks
(0% = low watermark, 100% = high watermark).

To that end, get_buffering_percent() is renamed to get_buffering_level(),
and the code at the end that transforms to the buffering percentage is
factored out into a new convert_to_buffering_percent() function. Also,
the buffering level range is parameterized by adding a new constant called
MAX_BUFFERING_LEVEL.

https://bugzilla.gnome.org/show_bug.cgi?id=769449