Added `on_gain_focus`, `on_lose_focus`, `on_show` & `on_hide` handlers on `toga.Window` #7807

Job	Run time
Pre-commit checks / Pre-commit Checks	35s
Check towncrier / Towncrier Checks	24s
Package Toga (android) / Create Python Package	26s
Package Toga (core) / Create Python Package	17s
Package Toga (cocoa) / Create Python Package	14s
Package Toga (demo) / Create Python Package	23s
Package Toga (dummy) / Create Python Package	13s
Package Toga (gtk) / Create Python Package	15s
Package Toga (iOS) / Create Python Package	14s
Package Toga (textual) / Create Python Package	24s
Package Toga (toga) / Create Python Package	22s
Package Toga (web) / Create Python Package	15s
Package Toga (winforms) / Create Python Package	16s
MicroPython compatibility	41s
Test core (macos-latest, 3.9)	1m 11s
Test core (macos-latest, 3.10)	1m 14s
Test core (macos-latest, 3.11)	41s
Test core (macos-latest, 3.12)	49s
Test core (macos-latest, 3.13)	52s
Test core (ubuntu-latest, 3.9)	50s
Test core (ubuntu-latest, 3.10)	51s
Test core (ubuntu-latest, 3.11)	49s
Test core (ubuntu-latest, 3.12)	57s
Test core (ubuntu-latest, 3.13)	1m 10s
Test core (windows-latest, 3.9)	1m 28s
Test core (windows-latest, 3.10)	1m 45s
Test core (windows-latest, 3.11)	1m 33s
Test core (windows-latest, 3.12)	1m 38s
Test core (windows-latest, 3.13)	2m 36s
Coverage	29s
Testbed (macOS-x86_64)	6m 59s
Testbed (macOS-arm64)	6m 27s
Testbed (windows)	6m 50s
Testbed (linux-x11)	4m 58s
Testbed (linux-wayland)	5m 22s
Testbed (android)	6m 58s
Testbed (iOS)	5m 58s
Testbed (textual-linux)	52s
Testbed (textual-macOS)	54s
Testbed (textual-windows)	1m 55s
	1h 11m 5s

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