WindowServer: In HighDPI mode, load high-res window buttons and high-res cursors

Bitmap::load_from_file("foo.png", 2) will now look for "foo-2x.png" and
try load that as a bitmap with scale factor 2 if it exists. If it
doesn't, it falls back to the 1x bitmap as normal.
Only places that know that they'll draw the bitmap to a 2x painter
should pass "2" for the second argument.

Use this new API in WindowServer for loading window buttons and
cursors.

As a testing aid, ctrl-shift-super-i can force HighDPI icons off in
HighDPI mode. Toggling between low-res and high-res icons makes it easy
to see if the high-res version of an icon looks right: It should look
like the low-res version, just less jaggy.

We'll likely have to grow a better API for loading scaled resources, but
for now this suffices.

Things to check:
- `chres 640 480` followed by `chres 640 480 2` followed by
  `chres 640 480`
- window buttons in window context menu (in task bar and on title bar)
  still have low-res icons
- ctrl-shift-super-i in high-res mode toggles sharpness of window
  buttons and of arrow cursorf
- arrow cursor hotspot is still where you'd expect

Userland/Services/WindowServer/WindowManager.h

@@ -230,6 +230,9 @@ public:
 
     Gfx::IntPoint get_recommended_window_position(const Gfx::IntPoint& desired);
 
+    int compositor_icon_scale() const;
+    void reload_icon_bitmaps_after_scale_change(bool allow_hidpi_icons = true);
+
 private:
     NonnullRefPtr<Cursor> get_cursor(const String& name);
 
@@ -263,6 +266,7 @@ private:
 
     void do_move_to_front(Window&, bool, bool);
 
+    bool m_allow_hidpi_icons { true };
     RefPtr<Cursor> m_hidden_cursor;
     RefPtr<Cursor> m_arrow_cursor;
     RefPtr<Cursor> m_hand_cursor;