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.
Signed-off-by: David Heidelberg <david@ixit.cz>
HdyDialog is an adaptive version of GtkDialog that's always modal and adapts to fit the size of the screen if folded with HdyLeaflet. In the process of changing to use HdyDialog the width_request of the dialogs was moved from the window to their HeaderBars.
Related to https://gitlab.gnome.org/GNOME/fractal/issues/377
