Split from: #4

Add an option to display the class icons on the bars.

Deferred due to the following reasons:

• adding class specialization icons (protection, arms, etc) needs to use Inspect API, which brings some complexity
• adding only class icons (warrior, druid, etc) don't really add new information (we already have class colored bars)

Further notes on implementation:

• add toggle in settings dialog for displaying or hiding the class icons
• find a clean design for displaying the icons which doesn't increase visual clutter