iconGrid: Add more possible icon sizes

When selecting the largest possible icon size for restricted vertical or
horizontal space the options for low resolution icons were rather coarse
grained. This could often result in seemingly too small icons being
chosen in the app grid on systems with low vertical resolution, because
the next larger size would exceed the limit by a few pixels.

This adds two more commonly used sizes for application icons to have
some more options with restricted space.

Helps: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2173
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2289>

js/ui/iconGrid.js

@@ -13,7 +13,9 @@ var PAGE_SWITCH_TIME = 300;
 var IconSize = {
     LARGE: 96,
     MEDIUM: 64,
+    MEDIUM_SMALL: 48,
     SMALL: 32,
+    SMALLER: 24,
     TINY: 16,
 };