External editing for layer content

Add UI and layer APIs to support editing layers via tempfiles, using
external applications. Currently only the default application for a given
file type can be launched, and only file-backed layers (e.g. VectorLayers)
can be edited this way (they benefit most).

Addresses #57.
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.
Support for vector layers (SVG, edited externally)

Adds the ability to create vector layers and edit them in Inkscape (if
that's your default SVG file opener). Some polishing may be needed, such as
offering a choice from the DE's recommended SVG apps, but for now this
should suffice and allow testing.

Fixes #57.

There seems to be a consensus on the CREATE mailing list that arbitrary
image-like (or otherwise) layer file data is welcome in OpenRaster
containers, and that this is the correct interpretation of the current
spec. SVG is one of the more useful of these formats, since we gain useful
text and graphics layers.

To use this feature, create a new vector layer, then edit it with your
favourite external app via the menu. When you save the temporary file, a
new entry is created on the MyPaint undo stack.

Rendering in MyPaint is ultimately via librsvg, meaning you may need to
simplify Inkscape SVG files which rely on Inkscape-specific extensions
quite a lot. If nothing displays at all, make sure your librsvg install
ships a libpixbufloader-svg.so and that gdk-pixbuf-query-loaders lists SVG.