Add missing infobar for backends

[diegogangl]

fixes #998

⚠️ This needs help testing!

[azmeuk]

The infobar exception is fixed, now I have:

Context: Global generic exception

Traceback (most recent call last):
  File "/home/eloi/dev/gtg/.local_build/install/lib/python3.13/site-packages/GTG/gtk/browser/main_window.py", line 1709, in remove_backend_infobar
    self.vbox_toolbars.foreach(self.__remove_backend_infobar, backend_id)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Box' object has no attribute 'foreach'

[pentux-GitHub]

I tested this PR and can confirm it fixes the original set_interaction_request
exception from #998. However, @azmeuk's report is correct — a secondary error
occurs immediately after:

AttributeError: 'Box' object has no attribute 'foreach'

This is because foreach() is a GTK3 method that no longer exists on GtkBox
in GTK4. It appears in two places in main_window.py, and vbox_toolbars.add()
has the same issue (GTK3 → GTK4).

Here is the fix for all three occurrences:

remove_backend_infobar() (line ~1708):

# Before
self.vbox_toolbars.foreach(self.__remove_backend_infobar, backend_id)

# After
child = self.vbox_toolbars.get_first_child()
while child:
    next_child = child.get_next_sibling()
    self.__remove_backend_infobar(child, backend_id)
    child = next_child

_new_infobar() (line ~1720 and ~1724):

# Before
self.vbox_toolbars.foreach(self.__remove_backend_infobar, backend_id)
...
self.vbox_toolbars.add(infobar)

# After
child = self.vbox_toolbars.get_first_child()
while child:
    next_child = child.get_next_sibling()
    self.__remove_backend_infobar(child, backend_id)
    child = next_child
...
self.vbox_toolbars.append(infobar)

Tested on the current master with a live CalDAV backend — the foreach
exception is gone after applying these changes.

Happy to open a follow-up PR if that's helpful.