BO: Modules - show update button even if module is not installed but don...

BO: Modules - show update button even if module is not installed but don't warn for modules with updates available but not installed.

From the idea brought by @PrestaEdit, in pull request #724, here's my proposal to allow modules that are not installed (but present on disk) to be automatically updated before being installed.

I too (like @PrestaEdit) believe it doesn't make sense to warn users for an available update for a module they're not using.