Added `on_gain_focus`, `on_lose_focus`, `on_show` & `on_hide` handlers on `toga.Window` #7810

Job	Run time
Pre-commit checks / Pre-commit Checks	38s
Check towncrier / Towncrier Checks	26s
Package Toga (core) / Create Python Package	15s
Package Toga (cocoa) / Create Python Package	14s
Package Toga (android) / Create Python Package	16s
Package Toga (dummy) / Create Python Package	14s
Package Toga (gtk) / Create Python Package	23s
Package Toga (iOS) / Create Python Package	14s
Package Toga (demo) / Create Python Package	24s
Package Toga (textual) / Create Python Package	16s
Package Toga (toga) / Create Python Package	20s
Package Toga (web) / Create Python Package	22s
Package Toga (winforms) / Create Python Package	16s
MicroPython compatibility	36s
Test core (macos-latest, 3.9)	1m 10s
Test core (macos-latest, 3.10)	1m 16s
Test core (macos-latest, 3.11)	44s
Test core (macos-latest, 3.12)	47s
Test core (macos-latest, 3.13)	55s
Test core (ubuntu-latest, 3.9)	55s
Test core (ubuntu-latest, 3.10)	58s
Test core (ubuntu-latest, 3.11)	48s
Test core (ubuntu-latest, 3.12)	58s
Test core (ubuntu-latest, 3.13)	1m 3s
Test core (windows-latest, 3.9)	1m 40s
Test core (windows-latest, 3.10)	1m 44s
Test core (windows-latest, 3.11)	2m 5s
Test core (windows-latest, 3.12)	1m 54s
Test core (windows-latest, 3.13)	2m 6s
Coverage	24s
Testbed (macOS-x86_64)	7m 52s
Testbed (macOS-arm64)	6m 21s
Testbed (windows)	8m 27s
Testbed (linux-x11)	4m 52s
Testbed (linux-wayland)	5m 20s
Testbed (android)	11m 8s
Testbed (iOS)	6m 27s
Testbed (textual-linux)	48s
Testbed (textual-macOS)	1m 6s
Testbed (textual-windows)	1m 32s
	1h 18m 14s

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