mainwindow: escape nameLabel
Fixes a warning with the 'Valve VR Radio & HMD Mic' device.
(pavucontrol:1519965): Gtk-WARNING **: 22:28:41.575: Failed to set text 'Valve VR Radio & HMD Mic' from markup due to error parsing markup: Error on line 1: Entity did not end with a semicolon; most likely you used an ampersand character without intending to start an entity — escape ampersand as &
Haven't checked too much if the escaped string leaks and whether that is an issue.