LibWebView+WebContent: Use Web::InputEvent for WebContent input IPC

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.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -492,6 +492,7 @@ set(SOURCES
     NavigationTiming/PerformanceTiming.cpp
     Page/EditEventHandler.cpp
     Page/EventHandler.cpp
+    Page/InputEvent.cpp
     Page/Page.cpp
     Painting/AudioPaintable.cpp
     Painting/BackgroundPainting.cpp