Transform

3 new transform tools :

• Flip horizontally/vertically (#204) (can also be applied to all frames/layers)
• Rotate clockwise/counter-clockwise (#209) (can also be applied to all frames/layers)
• Clone layer (#205)

Others

• Thanks to @etumyan for #225 : real-time update of FPS for animation preview
• Fix for #228 : slow zoom on firefox
• Fix for #189 : Move viewport using the middle mouse button