Implement the auto_update feature

process the auto_update string from local.json file and set it when necessary.

re #6923
Extend signature check4update returning list of downloaded entries

If the check4update has to download the new version of files, these files
are listed to provide feedback on what has been done.

re #6923
Update unit test for check4update

re #6923
Enable AutoUpdate from Script Repository Interface

re #6923
Creation of the UpdateScriptRepository Algorithm

This algorithm will update the ScriptRepository and return a TableWorkspace
with all the entries downloaded.

re #6923
Remove the check4update from the ScriptRepositoryView

The call for the update for the Script Repository will be triggered
from the UpdateScriptRepository Algorithm.

re #6923
Remove the creation of TableWorkspace

I've discussed with Martyn this feature and hi suggested to remove it.

re #6923
Improve the interaction with autoupdate column on Gui

re #6923
Forgot to finish one line

add ; at the end of one line. shame on me.

re #6923
Triggering Update Script Repository on Mantid Startup

The requirement were:
  - in a different thread (so not to block on network issues)
  - At Mantid initialization (so to check frequently)

In order to use a different thread, it was necessary to execute trigger
the algorithm after the initialization of event loop. So, a new slot was
added to ApplicationWindow that is the first thing to be executed after
starting the event loop. about2Start.

re #6923
Merge remote-tracking branch 'origin/feature/6923_implement_auto_update'