FS#10722: some improvements for gui/list.c
* Add function list_get_nb_lines and replace duplicated codes to calculate number of lines by it.
* Refine calculation of start_item in function gui_list_put_selection_on_screen.
- displays as many items as possible. hopefully fix FS#10054.
* Fix checking bounds in function gui_synclist_item_is_onscreen.
* Remove trailing spaces and fix indents.
Flyspray: FS#10722
Author: myself
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@23367 a1c6a512-1295-4272-9138-f99709370657