Add and remove palette entries

A surprisingly late addition, you can now add and remove palette entries, I haven't added the UI buttons for this, but you can use hotkeys.

There might be a bug somewhere, but I think I've squashed all of them for now.

To add a new entry: Press either + or numpad +. The color of the new entry will use the color of the selected entry, otherwise it will use the style picker's current color.

To remove an entry: Press either -, numpad -, delete, or backspace. Only the selected entry will be deleted, if none are selected then nothing will happen. You cannot delete the last entry in a palette (I might change that).