implement sample limits

[LesserChance]

• [x] The samplepad docs say that there's a limit of 512 files in the root directory, but does that affect playback, or just the ability to select all the wav files when editing a kit? if it affects playback, I should put in a heavy warning if there are too many files. If it just affects editing, who cares, remove the concept of limits.
• [ ] also the docs say "You can assign up to 48 MB worth of samples per kit. Also, please note that a sample's size will be rounded up when loaded (e.g., a 2.1 MB sample on the SD card will be treated as a 3 MB sample when it is assigned to a pad)" - warn if this will be violated before saving