Overhaul of SBBCheckbox

[smallTrogdor]

Maybe this got a little bit bigger than expected 👀

This PR aligns the SBBCheckbox and SBBCheckboxListItem to the design guidelines again, with backward compatibility (users might experience little layout changes).

closes #215

What this PR adds:

• The SBBCheckbox is painted using a CustomPainter in a check animated fashion as a person would draw it on a piece of paper - it uses the ToggleableStateMixin and the buildToggeable for the logic.
• The colors of the SBBCheckbox and the SBBCheckboxListItem are aligned to the design specs (including actually respecting some of the colors previously not used (disabled colors)).
• The boxed variant of the SBBCheckboxListItem is added.
• The trailingIcon including the button is changed a little bit (see documentation - users might experience this as a small breaking change).
• The loading variant of the SBBCheckboxListItem is added via a isLoading parameter - this displays an animated BottomLoadingIndicator sweeping through the tile
• The documentation of the SBBCheckbox and the SBBCheckboxListItem has been revised and completed
• The semantics have been actually implemented (semanticsLabel on SBBCheckbox), merged with all the semantics of the SBBCheckboxListTile
• The checkbox page in the example app has been fully updated to show all the variants.

The layout of the SBBCheckbox has been corrected.

This is the design spec:

This was the old implementation with two stacked rounded rectangles:

The new loading indicator:

Screen.Recording.2025-01-08.at.09.23.28.mov

[VanHoangTran]

great stuff