xdg_desktop_portal

Addresses incorrectly doing this by hand in this pr:
ubuntu/app-center#2098 (comment)

I've also prepped to release by adding to the changelog and version
bumping

• Add getHostPaths() to Documents portal.

• Add trash portal
• Add ActionInvoked signal to notification portal.

• Add XdgSettingsPortal.settingChanged stream.
• Add some stub portals that just get the portal version for now.

• Add Secret, Documents portals.
• Don't send empty reason by default on accounts portal.
• Remove accidental code added to accounts example.

• Add Background portal.
• Add Camera portal.
• Expose request exceptions in the API.
• Add getVersion() methods for each portal.
• Fix requests not being closed after a response.

• Add Account portal.

• Add FileChooser portal.

• Use a property for XdgNetworkMonitorPortal.status

• Use a stream for network status.

• Hide request and session API, use exceptions and streams instead.

• Fix handling of requests and sessions.
• Added NetworkMonitor portal.

• Added location portal.
• Add missing documentation.

• Added proxy resolver, open URI and email portals.
• Fix actions on notification portal.

• Added notification portal.

• Initial release.