Added `on_gain_focus`, `on_lose_focus`, `on_show` & `on_hide` handlers on `toga.Window` #7806

Job	Run time
Check towncrier / Towncrier Checks	23s
Pre-commit checks / Pre-commit Checks	34s
Package Toga (android) / Create Python Package	21s
Package Toga (cocoa) / Create Python Package	17s
Package Toga (core) / Create Python Package	16s
Package Toga (demo) / Create Python Package	15s
Package Toga (dummy) / Create Python Package	22s
Package Toga (gtk) / Create Python Package	22s
Package Toga (iOS) / Create Python Package	17s
Package Toga (textual) / Create Python Package	14s
Package Toga (web) / Create Python Package	23s
Package Toga (winforms) / Create Python Package	25s
Package Toga (toga) / Create Python Package	25s
MicroPython compatibility	42s
Test core (macos-latest, 3.9)	51s
Test core (macos-latest, 3.10)	1m 2s
Test core (macos-latest, 3.11)	36s
Test core (macos-latest, 3.12)	42s
Test core (macos-latest, 3.13)	40s
Test core (ubuntu-latest, 3.9)	42s
Test core (ubuntu-latest, 3.10)	43s
Test core (ubuntu-latest, 3.11)	45s
Test core (ubuntu-latest, 3.12)	45s
Test core (ubuntu-latest, 3.13)	53s
Test core (windows-latest, 3.9)	1m 16s
Test core (windows-latest, 3.10)	1m 35s
Test core (windows-latest, 3.11)	1m 40s
Test core (windows-latest, 3.12)	1m 24s
Test core (windows-latest, 3.13)	2m 16s
Coverage	0s
Testbed	0s
	21m 6s

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