Fix crash closing directory from the openfiles sidebar

Fix crash closing directory from the openfiles sidebar

When the openfiles sidebar shows documents as a tree, closing a
document can lead to sever re-layout of the view (e.g. collapsing
directory nodes together).  This makes walking the tree and closing
documents at the same time highly tricky, as nodes might be shifting as
we go.

This lead to invalid memory access, and unexpected results, when
closing some tree structures.

One example of Valgrind showing how bad things are:

    ==917061== Invalid read of size 8
    ==917061==    at 0x5B31345: g_node_nth_child (gnode.c:1052)
    ==917061==    by 0x50A7361: gtk_tree_store_iter_nth_child (gtktreestore.c:793)
    ==917061==    by 0x491DB76: on_openfiles_document_action_apply (sidebar.c:1330)
    ==917061==    by 0x491DC0C: on_openfiles_document_action (sidebar.c:1347)
    ==917061==    by 0x5A833AF: g_closure_invoke (gclosure.c:832)
    ==917061==    by 0x5A96075: signal_emit_unlocked_R.isra.0 (gsignal.c:3796)
    ==917061==    by 0x5A9CBF4: g_signal_emit_valist (gsignal.c:3549)
    ==917061==    by 0x5A9CDBE: g_signal_emit (gsignal.c:3606)
    ==917061==    by 0x50D7833: gtk_widget_activate (gtkwidget.c:7845)
    ==917061==    by 0x4F8A935: gtk_menu_shell_activate_item (gtkmenushell.c:1375)
    ==917061==    by 0x4F8AC70: gtk_menu_shell_button_release (gtkmenushell.c:791)
    ==917061==    by 0x4DFBCB3: _gtk_marshal_BOOLEAN__BOXEDv (gtkmarshalers.c:130)
    ==917061==  Address 0x9bcdc20 is 32 bytes inside a block of size 40 free'd
    ==917061==    at 0x484317B: free (vg_replace_malloc.c:872)
    ==917061==    by 0x5B3068B: g_nodes_free (gnode.c:123)
    ==917061==    by 0x5B3068B: g_node_destroy (gnode.c:143)
    ==917061==    by 0x50AA8F2: gtk_tree_store_remove (gtktreestore.c:1229)
    ==917061==    by 0x491F851: sidebar_openfiles_remove_iter (sidebar.c:959)
    ==917061==    by 0x491F8AE: openfiles_remove (sidebar.c:972)
    ==917061==    by 0x491FA6C: sidebar_remove_document (sidebar.c:1027)
    ==917061==    by 0x48D0B5B: remove_page (document.c:733)
    ==917061==    by 0x48D29C0: document_remove_page (document.c:787)
    ==917061==    by 0x48D29FC: document_close (document.c:695)
    ==917061==    by 0x491DAED: document_action (sidebar.c:1299)
    ==917061==    by 0x491DB47: on_openfiles_document_action_apply (sidebar.c:1322)
    ==917061==    by 0x491DB9E: on_openfiles_document_action_apply (sidebar.c:1332)
    ==917061==  Block was alloc'd at
    ==917061==    at 0x48407B4: malloc (vg_replace_malloc.c:381)
    ==917061==    by 0x5B2C678: g_malloc (gmem.c:130)
    ==917061==    by 0x5B45011: g_slice_alloc (gslice.c:1074)
    ==917061==    by 0x5B305BD: g_node_new (gnode.c:110)
    ==917061==    by 0x50AAF0B: gtk_tree_store_insert_before (gtktreestore.c:1375)
    ==917061==    by 0x491E725: tree_add_new_dir (sidebar.c:654)
    ==917061==    by 0x491E89A: get_parent_for_file (sidebar.c:840)
    ==917061==    by 0x491E995: sidebar_openfiles_add_iter (sidebar.c:869)
    ==917061==    by 0x491F5C7: sidebar_openfiles_add (sidebar.c:901)
    ==917061==    by 0x48D0CE4: document_create (document.c:662)
    ==917061==    by 0x48D3840: document_open_file_full (document.c:1349)
    ==917061==    by 0x48D3C81: document_open_file (document.c:914)

Fix this by decoupling tree walking from closing documents.  We now do
two passes: first we collect documents to work on walking the tree as
before, and only then we perform the action on each node of the list.

Fixes #3527.