widgets
This commit introduces "batches", which makes Raven hold off on doing
full reflows until the batch is ended. When a window is created, a batch
is automatically started to ensure that no useless reflows are done
during initialization. The batch is automatically ended in window.run().
Users of the library can create and end batches as well. This commit
also factors out automatic scaling based on text size into set_text for
buttons (should be done for labels as well).
