Core

The Update Wizards in the install tool lack of some things that might be helpful:

• A proper API to give the wizards a title
• A proper API to let the user know whether the update wizard should be shown
• A possibility to go to the next update wizard once you're done with one
• A better visible divider between each update wizard
• A nice introduction for people so they understand what the update wizard does.

Solution:

• Create an abstract base class that takes care of the basic functionality of an update wizard (it also has the documentation for the functions that can be implemented)
• Modify some markers to have a consistent look+feel when performing the updates.
• Create some texts and fields to have the "Go to next update wizard" button and the introduction text above all update wizards.

Of course there are more things that can be improved further.

• Clean API function to display whether an update wizard does dbQueries or not
• Clean API if the update worked or not, with visual feedback a la FlashMessages with different states
  (issue imported from #M16845)