Display text using a drawing area instead of using a label with CSS

This patch reduces time spent in libgtk-3.so by up to 65% and improves
overall performance by up to 20%.

It also improves text centering when the frequency and the governor are
diplayed in two lines.

Mitigates: #9
2 jobs for master in 1 minute and 2 seconds
Status Job ID Name Coverage
  Build
passed #11835
build

00:00:24

 
  Distcheck
passed #11836
distcheck

00:00:37