fractal-gtk: add helper text to Join room dialog UI
Since spec #room:server is not so widely used, I propose to add placeholder text, which can help newbies to make less mistakes.
In future, Fractal could check syntax directly and grey-out Join button and colorize input field to warn about incorrect URL.
I'm open to suggestions to improve it further. Right now I set placeholder text to "#example_room:example_server.org", anything better describing is welcome.
Signed-off-by: David Heidelberg david@ixit.cz