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/Libraries/LibGfx/Bitmap.h

@@ -112,7 +112,7 @@ public:
     static RefPtr<Bitmap> create_shareable(BitmapFormat, const IntSize&, int intrinsic_scale = 1);
     static RefPtr<Bitmap> create_purgeable(BitmapFormat, const IntSize&, int intrinsic_scale = 1);
     static RefPtr<Bitmap> create_wrapper(BitmapFormat, const IntSize&, int intrinsic_scale, size_t pitch, void*);
-    static RefPtr<Bitmap> load_from_file(const StringView& path); // FIXME: scale factor
+    static RefPtr<Bitmap> load_from_file(const StringView& path, int scale_factor = 1);
     static RefPtr<Bitmap> create_with_anon_fd(BitmapFormat, int anon_fd, const IntSize&, int intrinsic_scale, const Vector<RGBA32>& palette, ShouldCloseAnonymousFile);
     static RefPtr<Bitmap> create_from_serialized_byte_buffer(ByteBuffer&& buffer);
     static bool is_path_a_supported_image_format(const StringView& path)