LibGUI: Add Widget focus policies

Every widget now has a GUI::FocusPolicy that determines how it can
receive focus:

- NoFocus: The widget is not focusable (default)
- TabFocus: The widget can be focused using the tab key.
- ClickFocus: The widget can be focused by clicking on it.
- StrongFocus: Both of the above.

For widgets that have a focus proxy, getting/setting the focus policy
will affect the proxy instead.

Libraries/LibWeb/InProcessWebView.cpp

@@ -68,6 +68,7 @@ InProcessWebView::InProcessWebView()
 {
     set_should_hide_unnecessary_scrollbars(true);
     set_background_role(ColorRole::Base);
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
 
     m_copy_action = GUI::CommonActions::make_copy_action([this](auto&) {
         GUI::Clipboard::the().set_plain_text(selected_text());

Libraries/LibWeb/InProcessWebView.h

@@ -61,8 +61,6 @@ public:
 
     void set_should_show_line_box_borders(bool value) { m_should_show_line_box_borders = value; }
 
-    virtual bool accepts_focus() const override { return true; }
-
     GUI::Action& select_all_action() { return *m_select_all_action; }
     GUI::Action& copy_action() { return *m_copy_action; }
 

Libraries/LibWeb/OutOfProcessWebView.cpp

@@ -40,6 +40,7 @@ namespace Web {
 OutOfProcessWebView::OutOfProcessWebView()
 {
     set_should_hide_unnecessary_scrollbars(true);
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
     m_client = WebContentClient::construct(*this);
     client().post_message(Messages::WebContentServer::UpdateSystemTheme(Gfx::current_system_theme_buffer_id()));
 }

Libraries/LibWeb/OutOfProcessWebView.h

@@ -68,7 +68,6 @@ private:
     OutOfProcessWebView();
 
     // ^Widget
-    virtual bool accepts_focus() const override { return true; }
     virtual void paint_event(GUI::PaintEvent&) override;
     virtual void resize_event(GUI::ResizeEvent&) override;
     virtual void mousedown_event(GUI::MouseEvent&) override;