LibWeb: Change where content selection via mouse is allowed

Previously, only DOM nodes with `is_editable()` allowed selection via
the mouse. This had the unwanted consequence, that read-only
input/textarea elements did not allow selection.

Now, `EventHandler::handle_mousedown()` asks the node's non-shadow
parent element over the new virtual method `is_child_node_selectable()`,
if selection of the node is allowed.
This method is overridden for `HTMLButtonElement` and
`HTMLInputElement`, to disallow selection of buttons and placeholders.

Fixes #579

Userland/Libraries/LibWeb/HTML/HTMLInputElement.h

@@ -204,6 +204,8 @@ public:
     WebIDL::ExceptionOr<void> set_selection_end_for_bindings(Optional<WebIDL::UnsignedLong> const&);
     Optional<WebIDL::UnsignedLong> selection_end_for_bindings() const;
 
+    virtual bool is_child_node_selectable(DOM::Node const&) const override;
+
 private:
     HTMLInputElement(DOM::Document&, DOM::QualifiedName);