WelcomeScreen: Don't use bold label text and exclamation marks.

It doesn't fit the rest of the UI, and Bold fonts and exclamation
marks always make be feel being yelled at.

Change-Id: If8ca6ce9d54777e65dda4d0c3002c298a9717ea2
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>