Make it more obvious that you can connect to servers on remote:/ page

Right now it is not super obvious how you connect to a remote server
in Dolphin when you already know the URL. Users will go to the
"Network" item in the Places panel, but from there it is not totally
clear. The "Add Network Folder" icon in the view opens a complicated
wizard and it's also a bit of an odd UX to have it living in the view as
opposed to being a toolbar button.

Old hands and developers know that the URL navigator is, well, a URL
navigator, and as such, it accepts arbitrary URLs from any view.
However this may not be obvious to other more casual users, for
two reasons:
1. The URL navigator is in breadcrumbs view nearly all of the time and
   by default, so may users may not know that it can accept text at all
2. Even when it's displaying URLs, they are almost always local paths,
   so users may not make the connection that it can accept remote URLs
   rather that just local paths

To improve the discoverability of this feature, this commit makes the
following change:

When the view is displaying the remote:/ ioslave (i.e. the "Networks"
place), the URL navigator is put into URL entry mode and given some
placeholder text that hints at what it can do. It reverts to breadcrumbs
mode when you leave.

BUG: 414670
FIXED-IN: 21.04