Images on public sharing get downscaled to increase use experience - this will speed up loading time

- adding keep aspect to core/ajax/preview.php
- remove duplicate method Preview::show()
- no more hard coded mimetype of preview
- remove .png from the preview urls
- keep old route preview.png for backwards compatibility
- aspect preserving previews are now cached

- moved file selection code to FileList
- fix selection summary when all files are selected
- nextPage now auto-selects files if "select all" checkbox is checked
- fixed trashbin to use the same selection logic as FileList

- moved the summary code into a new class FileSummary
- FileSummary is calculated only once, then updated with add/remove
- added new OC.Util namespace for JS utility functions

to => too