Pre-empt the default URI scheme handlers, so that applications may
specify scheme, host, port and path patterns.

URIs are checked incrementally by part.

When matching URI handlers, prevent sending a link back to the source
application. This allows an application to define broad patterns and
reject unwanted URIs by explicitly calling `OpenUri()`.

This adds support for deserializing URI handlers from `.desktop`
overrides in the user data directory, with the format:

```ini
[URI Handler example.com]
Scheme=https;
Host=example.com;*.example.com;
Port=443;
Path=/resource/*
```

Add support for reading URI handlers from a proposed addendum to
the intents-spec in the form:

```ini
[Implements org.freedesktop.UriHandler]
Patterns=*.openstreetmap/node/*;*.openstreetmap/way/*;
```