When using the copy buttons from Desktop Sharing/Remote Login pages,
the toasts don't show that the values were copied to the clipboard,
unlike in System > Secure Shell/About > System Details, for example.
So, fix this inconsistency.
Right now we set a subtitle of "" when the port is invalid. This
leads to the "Port" title getting center aligned vertically.
We also keep the port sensitive even when it's empty and there is
little value in copying it.
This commit changes the subtitle to " " so it serves as a
placeholder to keep the "Port" title top aligned. This commit
also makes the port insensitive in that case so the user never
copies the placeholder space.
