Firefox in Flatpak sandboxed environment does not get the list
of installed applications on the system because application should
know about the environment as little as possible. Introducing
nsFlatpakHandlerApp which forwards requests for opening downloaded files
to the system by utilizing gtk_show_uri fuction.
This changeset also removes nsIGIOMimeApp::Launch method from the interface
because it can be fully replaced with LaunchWithUri from nsIHandlerApp
interface.
The TMPDIR where files are downloaded when user choose to open them
needs to be accessible from sandbox and host. The default settings
TMPDIR=/tmp is accessible only to the sandbox.
To workaround for is to set TMPDIR environment variable to
$XDG_CACHE_HOME/tmp before executing Firefox.
MozReview-Commit-ID: CSBv0QcETpd
d5b0cb0ab4