Add support for relative sprites in the animation tool

[remy]

Add support for relative sprites in the animation tool

[paulossilva]

Being more specific on how to add the relative sprite's attributes and suggesting an UI to handle them (apply to both composite and unified sprites):

1. Visibility would be just a checkbox to set whether a relative sprite is visible or not. Making an anchor sprite not visible makes all relative sprites invisible too regardless their visibility attribute (which has no practical effect on the animation tool).

2. X,Y coordinates are relative offsets (-127 to 127) from the coordinates of the anchor. UI could use a spinner input field.

3. Pattern number could be relative to the anchor or independent from it (so is the palette offset), depending on whether specific attribute bits are set or not. Considering the sprite animation tool is aimed to help to build animations but not to generate code for it, the UI could be simplified to use input fields for entering independent values from their anchor sprite. An extra mile target would be let the user select a checkbox to determine whether these values are relative or absolute.

For Composite sprites, the following attributes are all independent from the anchor sprite:

1. Rotation, X-mirroring and Y-mirroring are just flags and could use of a traditional checkbox.
2. X-scaling and Y-scaling factors should be one of the values 1x, 2x, 4x and 8x.

For Unified sprites, these attributes are controlled by the anchor sprite, so they can't be changed individually on each relative sprite. Thus, the UI could just disable these fields.