Remove View Cached Windows And Buffers

[Alexander Courtis]

View.tabpages and BUFNR_PER_TAB are cached an not always correct. See #2624

Remove the caches and always determine the actual.

Ensure no O(n) calls to View.

get_bufnr
is_visible
get_winnr
open_in_win
abandon_current_window
abandon_all_windows
_prevent_buffer_override