Global Portal support

# Here is an overview of the portal support :

All the portals are implemented by the xdg-desktop-portal-gtk that we are using. We can override some of them for the Pantheon session.

## Already overridden:
- [X] org.freedesktop.portal.FileChooser — File chooser portal : Overridden by the Files supplied one.

## Requires discussion:
- [ ] org.freedesktop.portal.Device — Portal for device access
- [x] org.freedesktop.portal.Screenshot — Portal for taking screenshots
- [x] org.freedesktop.portal.ScreenCast — Screen cast portal
- [x] org.freedesktop.portal.Location — Portal for obtaining information about the location
- [ ] org.freedesktop.portal.RemoteDesktop — Remote desktop portal
- [x] org.freedesktop.portal.Background — Portal for requesting autostart and background activity
- [ ] org.freedesktop.portal.GameMode — Portal for accessing GameMode
- [ ] org.freedesktop.portal.Camera — Camera portal
- [ ] org.freedesktop.portal.Documents — Document portal
- [x] org.freedesktop.portal.Wallpaper — Portal for setting the desktop's Wallpaper
- [ ] org.freedesktop.portal.FileTransfer — Portal for transferring files between apps

## No need:
- [X] org.freedesktop.portal.Notification — Portal for sending notifications
- [X] org.freedesktop.portal.Trash — Portal for trashing files
- [X] org.freedesktop.portal.OpenURI — Portal for opening URIs
- [X] org.freedesktop.portal.Print — Portal for printing
- [X] org.freedesktop.portal.Email — Portal for sending email
- [X] org.freedesktop.portal.Inhibit — Portal for inhibiting session transitions
- [X] org.freedesktop.portal.NetworkMonitor — Network monitoring portal
- [X] org.freedesktop.portal.ProxyResolver — Proxy information
- [X] org.freedesktop.portal.Settings — Settings interface
- [X] org.freedesktop.portal.MemoryMonitor — Memory monitoring portal
- [X] org.freedesktop.portal.Flatpak — Flatpak portal
- [X] org.freedesktop.portal.Flatpak.UpdateMonitor

## New possible portals:
- [ ] io.elementary.portal.Contractor — Portal for getting contracts and using them
- [ ] io.elementary.portal.OnlineAccounts — Portal for getting online accounts