Adds support for FileChooser Flatpak portal. To support them we just do
specific DBus calls, which are then caught and forwarded by xdg-desktop-
portal daemon/service. This is needed for Qt applications running in
sandbox.
[ChangeLog][Platform Specific Changes][Linux] Added support for flatpak
portals. Flatpak is a software utility for software deployment and
package management. It provides a sandbox environment in which users
can run applications in isolation from the rest of the system. To
communicate with the system flatpak uses portals, which are designed to
be a bridge between sandboxed applications and desktop/system running on
user's computer. Flatpak runs this service (called xdg-desktop-portal)
automatically. It exports the portals via DBus and they are visible by
default to all applications running under Flatpak.
Change-Id: I4de1402434ba7cbcc805eab51c30f84f8ba0c5c5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
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