Re-add WebImage

[PoignardAzur]

WebImage was removed for a few reasons:

• It was added for testing promises, but it wasn't really a core widget and it definitely wasn't important enough to justify the problems it created.
• It loaded its contents by calling reqwest::blocking::get(url) in a background thread, which isn't ideal for a few reasons:
• Bad performance.
• Adding an unnecessary dependency to reqwest that wasn't used anywhere else in the project.
• That specific function didn't compile on wasm.
• It relied on ImageBuf::from_data, which caused some build problems because that method wasn't reliably generated depending on feature flags.

I would like to add that type back eventually, with a few caveats:

• Wait for the transition to glazier (Huge memory consumption from parley::FontCache::new() (1.8 GiB) #24) where we'll hopefully have a better import structure.
• The type shouldn't use any network code itself. We should use dependency injection and pass a builder function as a parameter instead.
• We should implement running async functions in the background to avoid having to call reqwest::blocking.