[metif12]

Tests only. I gap-checked the menu bar and tray against what an embedder needs
and found no code gap, so this adds no API.

Stacked on #54, #55 and #56. GitHub will not accept a base branch that
exists only on the fork, so this PR necessarily targets master and its diff
includes the layers below. This PR's own change is the single commit f4f9397
— use the "Commits" tab and filter to it, or ask me for that commit's diff.
Once those merge I will rebase onto the new master so the diff is this layer
alone.

Windows scenarios covered, in menu_tray_test.v at the repository root (see the
native_window_handle PR for why the tests live there):

• a menu bar replaced at runtime — set_menu_bar twice, the second declaration
  wins
• a menu command reaching the app's event handler
• a tray tooltip updated after the icon is installed
• a tray icon clicked with no menu attached — left-click emits its id,
  right-click emits nothing

Plus a portable test that set_menu_bar/menu_bar round-trip the declaration,
which is what keeps this file valid on the backends where the Windows block
compiles out.

Verification

-check passes for windows native/custom, linux native/custom, android
native/custom. The file runs green on Windows, natively and with
-d ui2_custom_rendering. macOS and iOS are not verified — no macOS host
here.

CI is red on this branch for the pre-existing ui/chart.v int/float issue
described in the native_window_handle PR, not for anything here.