This change adds an `{exec_arg}` field to the default `main.desktop` template.
This field is populated by handlebars with a sane default value,
which is based on whether `deep_link_protocols` or `file_associations`
in the `Config` struct have been specified.

This allows an installed Debian package to be invoked by other applications
with URLs or files as arguments, as expected.

The `main.desktop` template previously did not automatically include
an Exec field code even if `deep_link_protocols` or `file_associations`
were set, which would confuse new users who incorrectly expected that
the Debian package would automatically handle links and file associations
upon installation. This change now ensures their expectations are met.

Co-authored-by: Amr Bashir <[email protected]>

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