Allow the use of toga.Window as the main window #6134

Job	Run time
Pre-commit checks / Pre-commit Checks	39s
Check towncrier / Towncrier Checks	31s
Package Toga (cocoa) / Create Python Package	17s
Package Toga (android) / Create Python Package	19s
Package Toga (demo) / Create Python Package	29s
Package Toga (dummy) / Create Python Package	19s
Package Toga (gtk) / Create Python Package	23s
Package Toga (iOS) / Create Python Package	20s
Package Toga (core) / Create Python Package	21s
Package Toga (textual) / Create Python Package	17s
Package Toga (toga) / Create Python Package	30s
Package Toga (web) / Create Python Package	29s
Package Toga (winforms) / Create Python Package	30s
Test core (macos-12, 3.8)	1m 11s
Test core (macos-12, 3.12)	1m 15s
Test core (macos-12, 3.13-dev)	1m 3s
Test core (macos-14, 3.12)	44s
Test core (macos-14, 3.13-dev)	39s
Test core (ubuntu-latest, 3.8)	52s
Test core (ubuntu-latest, 3.12)	1m 2s
Test core (ubuntu-latest, 3.13-dev)	33s
Test core (windows-latest, 3.8)	1m 40s
Test core (windows-latest, 3.12)	1m 31s
Test core (ubuntu-latest, 3.9, false)	53s
Test core (ubuntu-latest, 3.10, false)	45s
Test core (ubuntu-latest, 3.11, false)	53s
Coverage	26s
Testbed (macOS-x86_64)	3m 32s
Testbed (macOS-arm64)	2m 25s
Testbed (windows)	4m 54s
Testbed (linux-x11)	2m 35s
Testbed (linux-wayland)	3m 4s
Testbed (android)	7m 48s
Testbed (iOS)	4m 56s
	48m 5s

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