Sprite Editor: Add some basic drawing tools

[HomineLudens]

I think it would be nice allow user to:

• Draw lines (modifier key could snap to orthogonal)
• Draw rectangles (modifier key could force square size)
• Draw ovals (circle with modifier key)

[TinkerStudio]

Flip and rotate sprite would also be useful sometimes

[pmgl]

Here is the status on this one:

• Flip and rotate (selection) were added some time ago
• My last commit added the support for the Shift key modifier to draw a straight line from the last drawn point (drawing has also been improved by fixing #163)

The feature request is not fully addressed though so I am leaving it open...