Avoid resizing the CPU usage labels when their text changes

As the number of digits in the CPU usage labels vary, their width was varying too. When the window was narrow enough to be around the natural width of the graphs, this could cause the width of all the graphs on the Resources tab to resize.

Set a desired width of 6 characters on these labels. This is hopefully enough for all values ("100.0%" is probably the longest), preventing the size changing with the value, but it may vary depending on font.

Fixes #149 (closed), assuming the other regularly changing labels are not the limiting factor in the width of the Resources tab.