This replaces the previous Web::ImageDecoding::Decoder interface.
While we're doing this, also move the SerenityOS implementation of this
interface from LibWebView to WebContent. That means we no longer have to
link with LibImageDecoderClient in applications that use a web view.
Instead of hard-coding the names of system fonts to use for the CSS
generic fonts (like "sans-serif", "monospace", etc.) we now call out
to a Platform::FontPlugin and ask for the generic names.
Instead of using Core::EventLoop and Core::Timer directly, LibWeb now
goes through a Web::Platform abstraction layer instead.
This will allow us to plug in Qt's event loop (and QTimer) over in
Ladybird, to avoid having to deal with multiple event loops.
