Add support for local app bundles

[Chris Coulson]

Spun off from bug 1347892 (in particular, see https://bugs.launchpad.net/oxide/+bug/1347892/comments/8):

Local applications are currently loaded via file:/// URLs, but this presents some issues:

- Chromium considers all unique file paths as a different domain. Bug 1347892 was originally reported because a local application can't render an image in to a canvas without marking it origin-unclean (which means, it can't read the pixels back). There is a WebPreferences API to disable this security feature, but it puts all file:/// URLs in to the same origin which is sub-optimal. And we shouldn't require application developers to rely on a dangerous API by default.

- The Filesystem API is disabled for file:/// URL's, and we shouldn't think about enabling that. Origin separation for local storage when using file URL's is pretty broken in any case.

What I've been thinking about is having "application bundles" in Oxide. Applications would be accessed via application:// URL's, with each registered application having its own domain (eg, accessing index.html in application "foo" would be done by application://foo/index.html). Underneath, each application domain would map to a directory on the filesystem.

This type of scheme would mean we can properly enforce domain separation between applications (so, local storage can be used) and it would fix bug 1347892 too without having to rely on dangerous APIs