Fixes performance of the content proxy

Before, the content proxy would re-sort and re-filter its content after any change to its content's labels (e.g. if something is added/deleted to the content, or if a label changed). This ultimately lead to O(n^2 * log(n)) performance for adding n items. This change makes it so that the filteredContent and sortedFilteredContent properties are updated in a debounced way which means that we should basically have O(n * log(n)) performance (only the last sort matters).

@azirbel