Release 2.4

• Overhauled the Layers window and handling:
  - improved layout
  - reduced commands to the most useful, moved to main menu
  - added naming layers
  - added transparency slider
  - added blending modes: Normal, Multiply. Divide, Screen,
  Overlay, Hard light, Soft light, Vivid light, Pin light,
  Hard mix, Difference, Exclusion, Color dodge, Linear dodge,
  Color burn, Linear burn, Darken, and Lighten
• Improved Colors window
  - removed YIQ and YUV color modes
  - added CIELAB color mode
  - accept drag'n'dropped colors
  - set/show hex-value of current color
  - removed palette from main window
• Make current tool more obvious in Tools window
• Bufixes and polishing