Be smarter about URIs and file paths passed in the command line

[aperezdc]

When interpreting an URI parameter in the command line without the http:// or https:// prefixes, Dinghy could:

1. Use the parameter as a file system path. If a readable file exists, turn it into a file:// URI with an absolute path, and load that instead.
2. Otherwise, try adding http:// to the URI, and load that instead.

Just doing the two steps above would make things much more convenient and work in 99% of the cases because most services behind HTTPS have HTTP-to-HTTPS redirections.

[aperezdc]

Passing a path is already working, g_file_new_for_commandline_arg(), the only thing missing is to check for file existence, and trying to turn it into an http:// URI if there wasn't an URI scheme already there.