Fix IndexError at startup when all view plugins are hidden

[eduralph]

Fix IndexError at startup when all view plugins are hidden (#8796)

Root cause

When every view plugin is hidden in the Plugin Manager,
ViewManager.get_available_views() returns [], and views_to_show([])
collapsed to (0, 0, []) — claiming category 0 exists when it does not. At
startup init_interface then called goto_page(0, 0), whose
self.current_views[cat_num] = view_num indexed the empty current_views
list and raised IndexError: list assignment index out of range, crashing
Gramps before the window opened (#8796).

Fix

views_to_show is made total over the empty view set: with no views it now
returns (None, None, []) ("no category to select") instead of
(0, 0, []), and init_interface only navigates when a category exists, so
the app opens with no active page rather than crashing. The function is moved
into a new GTK-independent module viewmanagerutils.py so production and the
regression test exercise the same implementation. Two sibling entry points
that reach the same empty-view state independently of startup navigation are
guarded the same way: __change_page (reached via _post_load_newdb_gui
when a tree auto-loads with zero views) and config_view (the ConfigView
keyboard action) now return early when there is no page / no active page.

Verified against

• gramps/gui/viewmanagerutils.py:39-47 — new gi-free views_to_show
  returns None, None, default_cat_views when views is empty (target
  branch maintenance/gramps61).
• gramps/gui/viewmanager.py:80 — viewmanager re-imports views_to_show
  from viewmanagerutils, so the production startup path routes through the
  exact function the test drives (no parallel copy).
• gramps/gui/viewmanager.py:638-656 — init_interface unpacks the tuple
  and navigates only if current_cat is not None, leaving no active page on
  the empty set.
• gramps/gui/viewmanager.py:1051-1061 — __change_page returns early when
  self.pages is empty (get_current_page() returns -1 on an empty
  notebook, so self.pages[-1]/self.pages[0] would raise).
• gramps/gui/viewmanager.py:1515-1524 — config_view returns early when
  self.active_page is None, so the ConfigView action does not dereference
  None.

Test

gramps/gui/test/viewmanager_test.py (new) drives the production
views_to_show headlessly and asserts views_to_show([], use_last=False)
and views_to_show([], use_last=True) both return (None, None, []) — the
corrected, totality-restoring contract that prevents the startup IndexError.

Manual GUI repro on maintenance/gramps61: hide every "Loaded Plugins" view
entry (or list all view plugins under [plugin] hiddenplugins in
gramps.ini) and restart — pre-fix Gramps crashes with the IndexError;
post-fix it opens with no active page and no exception.