ENH: quickly reinstall previously installed extensions v2

Created by: mneugeba

This pull request is related to this PR (https://github.com/Slicer/Slicer/pull/632 - could be closed as PR includes all the features). It includes all the feedback from the hangout with JC as well as feedback from Ron from January (check for extensions on startup, allow for completely automatic hands-off installation of extensions if a new slicer build is installed and started for the first time). Details: https://www.slicer.org/wiki/Documentation/Labs/AutomaticUpdateAndInstallationFramework#Adaptations_.28March.29_according_to_input_from_JC_.28December.29_and_Ron_.28January.29

As it relies on tracking previous installations of extensions, the Slicer.ini needs to be adapted to emulate previous tracking. Example:

[ExtensionsHistory] Revisions\25844=ChangeTracker, SlicerToKiwiExporter, CleaverExtension Revisions\25843=AnglePlanesExtension, ABC, CurveMaker, SlicerToKiwiExporter Revisions\25842=AnglePlanesExtension, CurveMaker, CMFreg ScheduledForRemoval=

Adding this setting to Slicer.ini would lead to identifying ChangeTracker, SlicerToKiwiExporter, CleaverExtensions as previously installed extensions (prompt on startup, preselection in the qSlicerExtensionRestoreWidget that would also show details about the other extensions installed in older versions of slicer).