External layer editing: app chooser dialog; bugfix

Present a dialog asking which application to launch when attempting to edit
layer data in an external app.

No longer error out if the app used/chosen will only support URIs. Assume
the file:// URI it will be launched with will be sufficient.

Fixes #80.
New Vector Layer: use view area, immediate edits

Instead of using a 64x64 blank SVG document for new vector layers, create
them with the current MyPaint viewport outlined, and with a document size
that encloses that outline. Closer to what people probably want, hopefully.

Creating a new vector layer now launches the editor immediately.

Show ellipses for the New Vector Layer commands: they require user input
now.