Improve get license dialog: Add a section on how to look up price

[RealRaven2000]

On the license screen, a user who wants to renew / extend their license will not know about the pricing and can only check by going to the (fastspring) shopping site - via clicking the Green button [Extend License] / [Renew License]

On the screen the user may assume that the renewal is instant, without seeing the price. Therefore a section should be added that explains that the actual shop is online and will open in a browser window. The user should be told that the price depends on region and VAT and that they can look it up and decide by choosing an option from the bottom of the window.

[RealRaven2000]

Implemented in 5.9.1 released on 09/04/2023 (April)