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/LibVT/TerminalWidget.cpp

@@ -96,7 +96,7 @@ TerminalWidget::TerminalWidget(int ptm_fd, bool automatic_size_policy, RefPtr<Co
     , m_config(move(config))
 {
     set_override_cursor(Gfx::StandardCursor::IBeam);
-
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
     set_accepts_emoji_input(true);
     set_pty_master_fd(ptm_fd);
     m_cursor_blink_timer = add<Core::Timer>();

Libraries/LibVT/TerminalWidget.h

@@ -85,8 +85,6 @@ public:
     void paste();
     void clear_including_history();
 
-    virtual bool accepts_focus() const override { return true; }
-
     Function<void(const StringView&)> on_title_change;
     Function<void()> on_command_exit;