Added infinite scrolling (in-memory list)
Fix file summary to use the whole file list

- moved the summary code into a new class FileSummary
- FileSummary is calculated only once, then updated with add/remove
Fix file selection for infinite scrolling (WIP)

- 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