Add command line option to disable saving username/password credentials

[Phillipus]

From @jbsarrodie:

Make it possible to run Archi with a command line option (set in Archi.ini) to fully disable saving the username/password.

When set, this option should remove/gray out the saving the username/password option from the UI:

• Refresh/Publish/Online Merge actions (probably other occurrences)
• "Http authentication" and "Proxy > Requires authentication" Preference fields
• Authentication tab from Properties view

This option has to be set through Archi.ini because in an enterprise setup, Archi will be stored in a location not writable by user, so this option will be set at install time (through a custom packaged deployed by IT team).

[Phillipus]

Not needed now that we have #152