dialogs: Fixes and Improvements to Vala demo

[Diego-Ivan]

Including:

• Use C-Style casting
• Use string formatting in console logs
• Use async-yield instead of signal callbacks (as in the JS demo)
• Initialize objects with properties
• Add missing comments from the JS demo
• Small fixes for confirmation and advanced dialogs

[sonnyp]

Can you document the missing guidelines in https://github.com/workbenchdev/demos/blob/main/STYLEGUIDE.md#vala ?

[Diego-Ivan]

Sure, just sent a PR with the missing ones :)

[sonnyp]

Could you solve conflicts?

Sorry the Dialogs demo was renamed.