Draft: Added style tags to each button group

• All buttons do now have a added style tag, according to the purpose of that button.
• Every button group now has it's own CSS style tag, they are classified into the following groups:
  • Functions
  • Operator buttons ( +-x/ buttons )
  • Undo buttons
  • Clear buttons
  • Result buttons
  • % buttons
  • dot-point buttons (. button)

You can add the style tags in calculator.css, by adding css style tags from each group individually.

It is up to the team of Gnome to set the default used styles.

Have a nice day :) Dave