Dynamically allocate memory for the file selector to store the name pointers into. Also dramatically simplify the file selector's name management code. This has many effects:

* It allows the file selector to handle directories of more than 511 files and a parent directory entry. (Fixes GitHub issue #37).
* It allows the file selector to sort directories along with files, which is a more common use case (game folders that contain ROM hacks, plus single game files, are sorted by name as the user expects).
* The file selector now reports progress while loading a large directory.
* The screenshot slideshow function now shows an existing dedicated message, "No screenshots found", instead of crashing when the screenshot directory contains no screenshots.

Add Quicksort code from GameYob. See <https://github.com/Nebuleon/GameYob/commit/afbeeba>.

* Faster sorting in the common case where a directory's natural order is already sorted, using the isSorted function.
* Use the median-of-three algorithm first described by Robert Sedgewick to select the pivot. Causes less recursion in many cases.
* Associates file names with their is-directory flag, keeping both in sync.