Add various input dialogs

[Semphriss]

I haven't seem much discussion about whether those dialogs should be added to SDL; I open this PR also as a discussion as to whether or not they should be, and which ones are relevant to SDL. I presumed it would be easier to discuss with working code available.

Description

This PR adds the following input dialogs types:

• Text/password input
  
• Progress/loading bar
  
• Color picker
  
• Date picker
  

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These also seem to be available, but I'm not sure if they're globally useful:

• List selection dialogs
  
• Scale number picker
  

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As of opening the PR, only the Zenity implementation is supported. I chose to open this PR early to get feedback on the current API.

Should I add the two remaining dialog types? More generally, which ones are good to add in SDL?

If some would be good to add:

• Currently all dialogs take an SDL_PropertiesID argument; should I make a simple version for each, similarly to SDL_ShowSimpleMessageBox?
• Should I re-use the same property field macros for fields that are similar across dialogs? (such as a single SDL_PROP_DIALOG_TITLE_STRING for all dialogs)
• For the date picker, I did not re-use the existing SDL_DateTime struct because it seems to be a complete representation of a given timestamp (including the day of the week and the UTC offset), and I wasn't sure how to handle this. I probably should move the code to SDL_DateTime, but I would need guidance on how to design the API.

I should be able to take care of the Windows implementation, but I haven't yet gotten my hands on a new macOS device; help would be welcome.

Existing Issue(s)

Fixes #12792

[expikr]

Would it perhaps be more suited to a separate SDL-blessed library a la SDL_Image etc?

[slouken]

Input seems useful in the core library, but the others are probably specific to particular use cases. @expikr's idea of a separate dialog specific library seems like a good one.

[Semphriss]

I'll move the non-input dialogs out of the PR shortly, I'll try for this weekend. I plan to begin working on the external library afterwards.

@slouken is there a specific way you would like to proceed to create the new library? I can make a working base and hand it over to the SDL organization to discuss how to properly organize the repository.

[slouken]

We don't have time to manage another library, feel free to create and maintain it yourself on GitHub. You can use SDL_net as a model, I think it's the smallest of the satellite libraries.