WebAudio - Allow 16 buffers in memory when using 1 section per round

[Evert-R]

Is your feature request related to a problem? Please describe.
When using 1 section per round, allow 16 buffers in memory before clearing the buffers.
Now this is done on every round, so if certain sections are used more than once, they are reloaded for every round that they are used, which offcourse is bad for performance.

Describe the solution you'd like

• Set a maximum number of buffers we allow to be loaded before the buffers are cleared. I would suggest setting this to 16, as this number of simultaneous buffers in memory has already been tested in the matching pairs game.
  The webAudio.clearBuffers() will still be called before preloading the first section in a round, but will only clear the buffers if there are already 16 buffers loaded in memory.
• Before preloading a section, check if the buffer is already loaded, and skip the preload if this is the case. This will improve both the matching pairs, as well as the categorization exp.

Describe alternatives you've considered

Additional context
Related issues: #1006 #1003