clear cache

This PR was implemented to clear caches under one specific Account directory (the currently login account).

Seadroid caches files on external SD card, big size caches like photos or files are placed under their specific Account directory. This PR assumes that user want to clear those big size files because user may only want to upload them to the server.

`R.layout.dialog_delete_file` asks the user _Are you sure to delete this file or folder?_, which is not proper here.