testutils: wait for element to be displayed/hidden

* Added methods 'wait_element_displayed_with_timeout' and
  'wait_element_hidden_with_timeout' to wait for an element to
  be displayed, or hidden, before proceeding.
WebSubmit: DEMOART uses bibdocfile_managedocfile

* Use bibdocfile_managedocfile interface (in response element
  DEMOART_FILE) to upload files in DEMOART submission, instead
  of file upload field + FFT.
WebSubmit: web tests DEMOART and DEMOPIC

* Test file upload within the DEMOART (SBI) submission, using
  the bibdocfile_managedocfile interface in asynchronous mode.

* Test file upload within DEMOPIC (SRV) submission, using the
  WebSubmit Create_Upload_Files_Interface.py function.
BibDocFile: new web tests

* Test addition, revision and deletion of files via
  the BibDocFile web interface.