container: preserve user OnTappedBar when MultipleWindows wraps it

[LunatikDG]

Description:

MultipleWindows.setupChild() runs on every refreshChildren() call — i.e. on
Add, RaiseToTop and Refresh — and unconditionally replaced
InnerWindow.OnTappedBar with its own raise-to-top handler. Any callback the
caller had already set on the window (before or after it was added) was
silently discarded, so setting OnTappedBar before the window's first render
never survived.

Fix: capture the existing OnTappedBar once and wrap it (user callback first,
then the built-in raise-to-top), guarded by a private tappedBarWrapped flag
on InnerWindow so repeated Refresh()/Add() calls don't keep re-wrapping
it into an ever-growing closure chain that would call the user's callback
multiple times per tap.

Fixes #6433

Scope note

InnerWindow.OnDragged/OnResized are overwritten by the same setupChild()
in exactly the same way and have the identical latent bug — I left them out of
this PR to keep it scoped to the one reported symptom, and opened #6481 to
track fixing them separately.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.
• Any breaking changes have a deprecation path or have been discussed.
• Check for binary size increases when importing new modules.

[coveralls]

coverage: 59.852% (+0.03%) from 59.823% — LunatikDG:fix/multiplewindows-ontappedbar-overwrite into fyne-io:develop