Make the ClutterText and ClutterTexture from the status menu
button available to JavaScript, and from there improve the
font definition of the user name.
shell-status-menu.[ch]: Add public get_name() and get_icon()
functions that return the user name label and icon
texture, remove the markup from update_name_text().
panel.js: Set the font for the button consistently with that
of the other panel labels.
