add remote HTTPS support to Qt GUI

This is a rather naive implementation that copies parts of the
SESSION_REMOTE_HOST settings and replaces HOST with HTTPS,
basically. We pull some ideas from the SESSION_REMOTE_AUTH parameter
as well (because it's a boolean) and we otherwise do not really know
what we are doing here.

In particular, we didn't add a new commandline flag for this, as I am
not sure what it would be called.

This explicitely does *not* add GUI elements as those were found to be
too confusing, as the backend does not support HTTPS. See #4593 for
the details of that discussion.

I actually would have much rather this be turned in a single URL
instead of having flags, UI elements and settings for what is
ultimately just a string, but that is yet another yak to shave...

Closes: #1294

libtransmission/quark.h

@@ -288,6 +288,7 @@ enum
     TR_KEY_remote_session_enabled,
     TR_KEY_remote_session_https,
     TR_KEY_remote_session_host,
+    TR_KEY_remote_session_https,
     TR_KEY_remote_session_password,
     TR_KEY_remote_session_port,
     TR_KEY_remote_session_requres_authentication,