Now that all input events are handled by LibWebView, replace the IPCs
which send the fields of Web::KeyEvent / Web::MouseEvent individually
with one IPC per event type (key or mouse).
We can also replace the ad-hoc queued input structure with a smaller
struct that simply holds the tranferred Web::KeyEvent / Web::MouseEvent.
In the future, we can also adapt Web::EventHandler to use these structs.
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