Reverse sorting on the depot dialog

[Ranran]

I noticed that the sort option in the depot dialog in standard does not have a reverse sort button, I implemented it in extended, but I think the same one could be added to standard.

I think the following commits can be used as a reference, although the file structure is different from standard.
https://github.com/Ranran-the-JuicyPork/simutrans-extended/commit/e988e16bb6b4c7d74856f578d8baef6bfca7a1a6
https://github.com/Ranran-the-JuicyPork/simutrans-extended/commit/cbf4a0019b20406b51ae63c3c9ce2dee14ee01e0
There are a few lines that are irrelevant to the standard. Also, button_t sort_order declaration for the depot_frame.h is missing from those commits (because it is included in another commit).