Added RecyclerView to build.gradle
Reduced number of jobs to one

This avoids memory spikes when more than one image is decrypted at the
same time.
Fixed NPE when deleting file without thumbnail
Fixed NPE when loading file with missing header
Removed onCreate from FileViewer

onCreate seems to be never called. It further would not work anymore
because opening a file without a loaded vault would not work.
Changed list_file layout to RecyclerView
Unregister VaultListFragment from EventBus when destroyed
Changed FileList to Recyclerview