Implement file forwarding for flatpak_run_app

Add a flag that triggers extra processing of the 'rest'
commandline arguments, as follows: When we see arguments
that are enclosed in a pair of '@@' arguments, we interpret
them as file paths, export them in the document store, and
pass the resulting document path to the launched application.

Currently, the files are exported non-persistent (i.e. only
for the current session), and with read and write permissions
for the app.
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