Directories with many files have significant performance issues when
generating thumbnails. This adds a preference that disables thumbnails
when the directory exceeds the threshold.

Fixes #1304

Valgrind shows:
==2022== 129,024 bytes in 504 blocks are still reachable in loss record 1,798 of 1,825
==2022==    at 0x4C29BC3: malloc (vg_replace_malloc.c:299)
==2022==    by 0xDDD860A: ??? (in /usr/lib64/libpixman-1.so.0.34.0)
==2022==    by 0xDD9F719: ??? (in /usr/lib64/libpixman-1.so.0.34.0)
==2022==    by 0x797DF88: ??? (in /usr/lib64/libcairo.so.2.11512.0)
==2022==    by 0x797EBE6: ??? (in /usr/lib64/libcairo.so.2.11512.0)
==2022==    by 0x797EC7B: ??? (in /usr/lib64/libcairo.so.2.11512.0)
==2022==    by 0x795224C: cairo_surface_create_similar_image (in /usr/lib64/libcairo.so.2.11512.0)
==2022==    by 0x6B88AB0: gdk_window_create_similar_image_surface (in /usr/lib64/libgdk-3.so.0.2200.30)
==2022==    by 0x6B61FDF: gdk_cairo_surface_create_from_pixbuf (in /usr/lib64/libgdk-3.so.0.2200.30)
==2022==    by 0x5157D2: caja_icon_info_get_surface_nodefault_at_size (caja-icon-info.c:576)
==2022==    by 0x52830A: get_action_icon (caja-ui-utilities.c:160)
==2022==    by 0x528691: caja_action_from_menu_item (caja-ui-utilities.c:189)
==2022==    by 0x4853EE: add_extension_menu_items (caja-window-menus.c:1097)
==2022==    by 0x485DEC: caja_window_load_extension_menus (caja-window-menus.c:1174)

gtk_action_group_add_action_with_accel() adds a reference to the action,
so we need to unref it after. The menu object is obtained via g_object_get()
which returns a full reference that must be unreleased.

Valgrind shows:
==2022== 40,416 bytes in 1,010 blocks are still reachable in loss record 1,771 of 1,825
==2022==    at 0x877258A: g_type_create_instance (in /usr/lib64/libgobject-2.0.so.0.5600.1)
==2022==    by 0x87561FC: ??? (in /usr/lib64/libgobject-2.0.so.0.5600.1)
==2022==    by 0x8758120: g_object_new_valist (in /usr/lib64/libgobject-2.0.so.0.5600.1)
==2022==    by 0x8758468: g_object_new (in /usr/lib64/libgobject-2.0.so.0.5600.1)
==2022==    by 0x468B2C: caja_notes_viewer_init (caja-notes-viewer.c:339)

gtk_text_view_new_with_buffer() adds a reference to the text buffer,
so we need to unref it after to transfer ownership.

The lookup key used size * scale while the insert key used original size,
causing cache misses and unbounded cache growth.

Destroy popup menu in finalize function since it's a floating widget
without parent. Also nullify borrowed pointers to avoid dangling references.