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.

Meta/gn/secondary/Userland/Libraries/LibWeb/Page/BUILD.gn

@@ -7,6 +7,7 @@ source_set("Page") {
   sources = [
     "EditEventHandler.cpp",
     "EventHandler.cpp",
+    "InputEvent.cpp",
     "Page.cpp",
   ]
 }