Adds an in-memory cache for samples. Callers can fetch data from the cache using two overloads: one for audio files, and another for Base64 strings. The cache stores weak pointers to the samples and returns shared pointers to the callers. However, the cache in the future will probably need to store the weak pointers along with sample thumbnails and possibly other kinds of metadata for each sample, which should be fairly easy to do as all that needs to be done is to store a collection of the data we need in a struct/class instead of just the weak pointer to the buffer.

For audio files, we check the last write time for the file to see if it needs updating each time it is fetched, and update it if necessary. For Base64 strings, we just compare the contents.

SampleLoader loads audio data by querying the cache for it, rather than creating them manually. As such, the SampleLoader::create* functions where renamed to SampleLoader::load*.

The memory usage has dropped significantly when using duplicate samples (htop readings went from 28.5% to 3.6% for me with a project that had a number of sample clips, each ~3 minutes long), and projects load faster (There is still some delay because of the waveform drawing, but this should be addressed soon. The speedup is more noticeable when you zoom all the way out before loading a project as a result).

Should supersede #7058 I believe.