add a layout debug mode that sets background colors #7331

Job	Run time
Check towncrier / Towncrier Checks	35s
Pre-commit checks / Pre-commit Checks	42s
Package Toga (cocoa) / Create Python Package	15s
Package Toga (android) / Create Python Package	17s
Package Toga (core) / Create Python Package	15s
Package Toga (dummy) / Create Python Package	14s
Package Toga (gtk) / Create Python Package	15s
Package Toga (iOS) / Create Python Package	15s
Package Toga (textual) / Create Python Package	15s
Package Toga (toga) / Create Python Package	18s
Package Toga (web) / Create Python Package	17s
Package Toga (demo) / Create Python Package	16s
Package Toga (winforms) / Create Python Package	18s
Test core	0s
Testbed	0s
Coverage	0s
	4m 12s

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