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 !32 with label-no-css in 1 minute and 4 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #11833
build

00:00:27

 
  Distcheck
passed #11834
distcheck

00:00:37