Handle fallback from short_title to title in model.

[danielballan]

In the views, we fall back and use title if short_title is None. Example:

bluesky-widgets/bluesky_widgets/jupyter/figures.py

Lines 80 to 97 in 7dbe644

	def _on_short_title_changed(self, event):
	"This sets the tab title."
	figure_spec = event.figure_spec
	widget = self._figures[figure_spec.uuid]
	index = self.children.index(widget)
	# Fall back to title if short_title is being unset.
	if event.value is None:
	self.set_title(index, figure_spec.title)
	else:
	self.set_title(index, event.value)
	def _on_title_changed(self, event):
	"This sets the tab title only if short_title is None."
	figure_spec = event.figure_spec
	if figure_spec.short_title is None:
	widget = self._figures[figure_spec.uuid]
	index = self.children.index(widget)
	self.set_title(index, event.value)

This should probably be moved to the model because:

1. It's duplicated in the various views.
2. We might want to make it fall back even further to axes_spec.title if figure_spec.title is None.