Workflow improvements

[seece]

The addon could be easier to use with the following changes (see forum post):

• Angle and frame names are now determined automatically. Didn't make sense to expose those data for the user to tinker with.
• Each mesh to be rendered now has a new property, SpriteName, to determine file name output.
• Support for more than 26 frames; in such a case, only the first 3 letters of SpriteName will be used. The 4th letter will be substituted with a number that increases every 26 frames.
• Added property (set on the Empty parent) for mirrored sprite names, and single angle sprites.

These are already implemented in Nash's fork but it only works in Blender 2.79.