AK: Use size_t for the length of strings

Using int was a mistake. This patch changes String, StringImpl,
StringView and StringBuilder to use size_t instead of int for lengths.
Obviously a lot of code needs to change as a result of this.

Libraries/LibGUI/GWindow.h

@@ -175,7 +175,7 @@ private:
     bool m_show_titlebar { true };
     bool m_keybind_mode { false };
     String m_entered_keybind;
-    int m_max_keybind_length { 0 };
+    size_t m_max_keybind_length { 0 };
     HashMap<String, WeakPtr<GWidget>> m_keyboard_activation_targets;
     bool m_layout_pending { false };
 };