PB Nextcloud Client 1.02

Feature: #27 Sort items by date modified (to choose ordering go to menu -> Order items by)
Feature: Show Modified difference to now and not the complete date
Feature: Show Message for local files on sync, user can select to delete files or keep them. Koreaders ".sdr" files will be ignored.
Feature: Introduce Itemstatus "Downloaded" to separate synced (only the structure in the DB is available) vs. Downloaded (files have been downloaded using sync method). Including a check if downloaded status still is true or files have been removed locally.
Feature: #26 Add option to ignore cert validation