Manage viewer binary out-of-band

[davidben]

The viewer process is currently managed by the system package manager. Unfortunately, Linux package managers are somewhat blind to the requirements of the application, and viewer binaries get updated without regard for whether they're still used. This results in a window in which wrapped plugins are broken because no one ran the updater. This is very error-prone and, especially with Ubuntu diverting Flash for some alternatives thing, we get problems.

Instead, don't reference the system-installed viewer when building a wrapped plugin. Copy it elsewhere and garbage-collect viewers as all wrapped plugins get updated. The nspluginwrapper package then exists to install a template for creating wrapped plugins, rather than providing pieces of one.

This probably depends on #4.