UI: Add shadow to panels

At the moment, only menus and tooltips have a shadow. However, other
elements can benefit from having a shadow. This PR adds shadows to
panels in overlapped regions.  Makes the default shadow size slightly
larger, but less opaque so it's not so prominent. While dragging a
panel this makes the shadow larger for a "floating" effect.

Pull Request: https://projects.blender.org/blender/blender/pulls/139847

release/datafiles/userdef/userdef_default_theme.c

@@ -239,8 +239,8 @@ const bTheme U_theme_default = {
       .blend = 0.5f,
     },
     .widget_emboss = RGBA(0x00000026),
-    .menu_shadow_fac = 0.4f,
-    .menu_shadow_width = 2,
+    .menu_shadow_fac = 0.2f,
+    .menu_shadow_width = 6,
     .editor_border = RGBA(0x161616ff),
     .editor_outline = RGBA(0xffffff15),
     .editor_outline_active = RGBA(0xffffff2a),