Add theme fields for 9-patch slice sizes

[treeform]

Summary

• Move hardcoded 9-patch border sizes onto Theme so a client can match its own atlas art.
• Widgets, textboxes, menus, and the DSL now read windowPatch, headerPatch, framePatch, and the other slice fields.
• Defaults stay the current values, so existing Silky examples keep the same look.

Test plan

• Run a Silky example (examples/basicwindow or examples/panels) and confirm windows, buttons, and scrollbars still slice correctly.
• Set sk.theme.windowPatch / framePatch to a different radius and confirm the Debug/subwindow chrome updates.
• Confirm Polyworld editor 9-patches work after applyEditorPatches.

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