Add API for flashing transient messages

Using the statusbar only for now, but there's no reason other mechanisms
couldn't be used in fullscreen mode.
API: expose the model's working tempdir

Expose the document model's tempdir as a public read-only property.

(Replace a minor deprecated call too)
App launching: better messages, more sensible EAFP

Display messages about external apps starting or failing to start via the
transient mechanism.

No method calls in try/except blocks when using EAFP quack checks for
the method's existence.
Add Vector Layer commands

Add commands to create vector (SVG) layers.

Closes #57.