Compute actual size of GUI objects lazily

This shifts the responsibility of updating the actual size more towards
IGUIObject, and enables only ever doing it when the value is actually
needed. This allows us to remove the delay of size changed
notifications, since the value is now already recalculated as
infrequently as possible anyways.

All of that ensures that the actual size (returned by GetActualSize) is
always up-to-date e.g. when reading it from the parent, which was
previously broken.

Fixes #8200

binaries/data/mods/_test.gui/gui/settings/cguisize/getcomputedsize.js

@@ -0,0 +1 @@
+testObject.getComputedSize();