Rename font_name variables/methods to reflect the fact its not just the name but includes the size
Tidyup
Fix overwriting saved text viewer font and size when preferences dialog is shown

When preferences dialog is shown, any saved text viewer font details
were overwritten with the default font and size - this bug was present
in the original code

Fixes #6 / https://github.com/OmegaPhil/hardware-monitor-applet/issues/6