Update the titles of the message windows in the Add-on Store (#16339)
When migrating an add-on update for which the version cannot be compared, the title of the confirmation dialog is "Add-on not compatible"; this is somewhat confusing because the add-on being installed may be compatible.
Description of user facing changes
Changed the title of this dialog with "Install add-on", as it is done for the "Remove add-on" action.
While at it, changed the title of other confirmation dialogs in the add-on store with the action being performed.
Description of development approach
Changed the strings in the code.