Problem: More functions can be used as methods.
Solution: Make various functions usable as a method.
f92e58cadb
Cherry-pick s:normalize_fname for tolower test from v8.1.0894 and v8.1.1417 (even though it is
unused for now).
Fix header for win_id2tabwin in eval.txt.
Problem: Cannot use a window ID where a window number is expected.
Solution: Add LOWEST_WIN_ID, so that the window ID can be used where a
number is expected.
888ccac890