Support updateable / removable extensions

As a Qt Creator user I want to

• understand which version of an extension is available / I have installed
• understand whether there are new versions for the extensions I am using
• update a previously installed extension
• remove a previously installed extension 

UI changes

display extension version 

• card
• header
• details (not needed? the header is always visible, this is not)

display whether there’s a new version of the extension available 

• overlay on the icon
• what text
  • needs to be short
  • needs to be different from "new"
  • suggestion: "update"? 

dynamic buttons 

• located in the header only 
• there’s always 1 or 2 buttons available for the user
• button to install an extension [exists already]
  • shown when the extension is not installed
• button to uninstall an extension
  • shown when the extension is installed
  • confirmation popover?
• button to update an extension
  • shown when the extension is installed, and there’s a new version available
  • confirmation popover? 

Data model changes

• track dependence to Qt Creator version? 

Documentation changes

• update extension
• remove extension 

Futureproofness