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, caches of big size like photos or files are placed under 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.

`getCacheSize` calculates the total size of the cache folder. This may take quite some time, so it should be done in an `AsyncTask` or  to avoid blocking the ui.